Brymen BM789

[Neutrion]

Hi everyone, finally I had to register myself to clarify something, but was reading the forum for a
while, so not a huge loss...
I am starting a new one for the BM789 as it does not seems to have an own topic yet.

So I got the bm789 with FW. version 08. (The Brymen multimeter, not the other bm789 equipment one can find on youtube,of which I was hoping to see an in depth review, done by the lady presenting it, after accidentally clicking on it, and watching the whole presentation.)

So I found two problems:

1.If I use the mV Scale and feed in a squarewave with 50% duty cycle without(or very minimal like 100-200mV) zero crossing (though it behaves almost the same way with less or more than 50%) something above 1 V p-p (so above the specs for the mV scale) it reads around
650mv on the AC scale, and it stays so even if I raise the voltage to around 30 V .(Can't get further)
 DC or AC+DC overranges properly.
If I change the duty % it will be a bit less than 650mV, around 500mV AC. So it peaks at 50%

The strange thing is, that if I chane the polarity of the leads, it behaves correctly and overranges around 650 mV AC which it should. So with one way the leads connected it behaves as expected, but not the other way around. I assume it depends on which polarity the squarevawe has.

Do you thing this is normal or acceptable? Before anyone mentions it, I know that I should use the higher ranges for signals like this, but when someone measures a lower level signal, and it raises up
above the limit, there is no way of knowing it, you just stop seeing anything above this level, while the voltagemight be skyhigh. I can not test it with higher voltages unfortunately, but would be nice if someone could do that.

So I am not vorrying about accuracy above the specified limit, but about the fact that it does not show overrange on the AC scale on the mV scale.

How do other meters behaves in this scenario?


2. Thats a minor issue, but still annoying: When measuring milli or micro amps, if on auto range, the measured currentis above the displayed range, so the meter has to switch range, on AC+DC mode first it displays "OL" for a second or more and only if you keep the leads connected it displays the proper values.
I thought, it is just a question of a timeout value in the FW, but Brymen says it is not possible to change it.
Should I keep it or not?


Ps:
Thanks to Dave for the inspiring Videos!

And also thanks to Joe Smith for your extensive work, wish we could see this level of testing with some more consumer goods as well, it would mean much less garbage be produced!

I would be especially interested in your oppinion on this, and whether it is a firmware bug or maybe hardware related.
I contacted Brymen through the seller, but they say it is all good and fine with the meter.



 

[2N3055]

mV range on these meters is NOT AC coupled. It is DC coupled.
So what you see is because of that.
You can check it with another meter, just measure resistance of input on BM789.

On my BM869S (that is same in that regard) I usually keep it in DC mV mode, and put AC and DC dual display most of the time.
That enables me to see more at the same time. But BM869S has dual display.

You could also take a 1uF block capacitor and put it in series with meter probe. That would AC couple the meter on mV range.
V AC, OTOH, is AC coupled...

[Neutrion]

But with the dual display you see at least that it overranged. But with the 789 if I don't check it all the time in dc or ac+dc it could mean the voltage is sometimes all over the place and I don't even see it.

How does this capacitor trick affect the accuracy in dc or ac+dc modes? Did you check the whole bandwith with it?

How do other meters cope with this?

[2N3055]

But with the dual display you see at least that it overranged. But with the 789 if I don't check it all the time in dc or ac+dc it could mean the voltage is sometimes all over the place and I don't even see it.

How does this capacitor trick affect the accuracy in dc or ac+dc modes? Did you check the whole bandwith with it?

How do other meters cope with this?

Dual display is the point. That is why I insist that single display meters are less capable than dual display.
Simple is not simpler all the time.

To me simplest meter would have big screen with simultaneous DC component, AC component (RMS), AC+DC (True RMS combined), frequency, duty cycle and accuracy limits. All at the same time. That would be simplest meter to use. You would have position for V, A, Ohms, Diode test. In Ohms, it would show resistance, voltage over resistor, current used and also accuracy limits.

Connecting through the capacitor is not a trick. This is what you do when you want to AC couple signal (strip it from DC component). DC component will be affected, of course, i.e. it won't be measured at all. Only AC will be passed, and that one will be pretty much perfect in a frequency range of the meter. It will have high pass effect. 1 uF will have impedance of 3180 Ohm at 50 Hz. That will make a divider with input impedance of meter (10MOhm + 600 pF for mV range, that is 3,466 MOhm combined at 50 Hz.). That makes it 1/1000 of influence, 0.1 %. That is one and half order of magnitude less then meter specs in that range. So simply ignore it. At 100KHz, parasitics will be negligible, so high end won't have any influence.

Draw a circuit in Ltspice and play with it. That is a good way to a get feeling for how it works.

[Neutrion]

Well yes dual display is alway better so I was also thinking a lot about the other Brymens. But than finally this became the choosen one. Was waiting on Joes channel for it to give me a final go, but he also not tested this thing.

Connecting through the capacitor is not a trick. This is what you do when you want to AC couple signal (strip it from DC component). DC component will be affected, of course, i.e. it won't be measured at all. .

Oh yes, I read parallel first, but that might wouldn't make any sense But thanks for the calculation, though this way I ended up having an ac only range.

But anyway, there are obviously ways around, but do you have exact the same issue with the 869s?
Also the second issue with the ac+dc mA measurement? Cause that one would be easier to solve in FW I suppose.

[bdunham7]

How do other meters behaves in this scenario?

OK, so I'm not clear on your exact signal.  You didn't specify the frequency, so I'm trying 100Hz and 1kHz.  As for the squarewave, am I correct in understanding that you have a 'DC' squarewave, IOW, the low is ~0V and the high is ~1V (or whatever).  So on a Fluke 115 116, with a 1-volt top squarewave I get OL on the 600mV AC range and ~500mV on DC, which seems correct.  With 500mV top voltage, I get ~350mV on AC and ~250mV on DC, which is also correct.  Reversing the polarity did not change the results. 

I can try other meters if you like.  If I misinterpreted your signal characteristics, let me know and I'll try whatever you actually used.

I didn't fully understand your second issue.  Is it just that the meter displays 'OL' for a time before settling on a correct reading, or is it something else?

[Neutrion]

Oh yes the frequency... so depending on the duty % up to about 500Hz (from 40) the problem is there, but above that it seems to disappear, and it overranges. And as I said, I could not sweep it through with higher voltages. And yes, all others as you just wrote.

But thanks for the test! 

Regarding the mA scale, yes, there is only the short "OL" which is annoying, because you would have to keep the probes on for 2-3 seconds longer to make sure it is settled.

[bdunham7]

Oh yes the frequency... so depending on the duty % up to about 500Hz (from 40) the problem is there, but above that it seems to disappear, and it overranges. And as I said, I could not sweep it through with higher voltages. And yes, all others as you just wrote.

Regarding the mA scale, yes, there is only the short "OL" which is annoying, because you would have to keep the probes on for 2-3 seconds longer to make sure it is settled.

The first does seem to be a bug to me.  I'm not sure what causes it internally, but it must have something to do with the way the chipset looks at AC.  External AC coupling may 'fix' it, but you shouldn't need to.  Many meters are 'DC-coupled' at the input stages even on AC ranges--the F115 116 I just used for example is ~10M on all ranges AC or DC, except 6V which is ~11M.

The second is just life with autoranging and doesn't seem to me to be an issue.

[2N3055]

Oh yes the frequency... so depending on the duty % up to about 500Hz (from 40) the problem is there, but above that it seems to disappear, and it overranges. And as I said, I could not sweep it through with higher voltages. And yes, all others as you just wrote.

Regarding the mA scale, yes, there is only the short "OL" which is annoying, because you would have to keep the probes on for 2-3 seconds longer to make sure it is settled.

The first does seem to be a bug to me.  I'm not sure what causes it internally, but it must have something to do with the way the chipset looks at AC.  External AC coupling may 'fix' it, but you shouldn't need to.  Many meters are 'DC-coupled' at the input stages even on AC ranges--the F115 I just used for example is ~10M on all ranges AC or DC, except 6V which is ~11M.

The second is just life with autoranging and doesn't seem to me to be an issue.

It's not a bug. If you have DC signal with AC superimposed on top, you might get to the point where signal is DC pulsating between two levels, one of which goes over ADC range. But still might be too fast switching for meter to up range... Autoranging is not magical.. There are scenarios where you can confuse it.

[bdunham7]

It's not a bug. If you have DC signal with AC superimposed on top, you might get to the point where signal is DC pulsating between two levels, one of which goes over ADC range. But still might be too fast switching for meter to up range... Autoranging is not magical.. There are scenarios where you can confuse it.

The OP may be able to clear this up, but I didn't see that it was specifically an autoranging issue, more that the meter didn't read OVERLOAD when it was clearly overloaded.

Now the DC vs AC input is an interesting characteristic. There is the external impedance and then there is how things are coupled and measured internally.  For the Fluke 116, for example, VAC and mVAC are both DC-coupled inputs, but internally it appears that VAC is AC-coupled whereas mVAC is not.  So given a 0-1V square wave, the 600mVAC range displays 'OL' but the VAC range correctly displays ~0.500V.  And given a 0-500mV square wave, the 600mVAC range displays ~350mV but the VAC range displays ~0.250VAC.  So mVAC is TRMS AC+DC.  And a closer look at the front panel reveals that I should have known that, but I didn't.   



Now the OPs meter gives the option of DC, AC and DC+AC.  It functions properly on DC and DC+AC, but fails to uprange or indicate overrange on the AC scale, and then only in one direction.  That seems wrong to me.  Thinking about it, perhaps there is some clamping bit that is breaking down too soon and actually keeping the sensed voltage from reaching the overrange threshold.  If some other EEVBlogger has a 786 or 789, perhaps they could help the OP out by running this test and seeing if it is a bug/feature or if the OPs unit has a defect.

[2N3055]

It's not a bug. If you have DC signal with AC superimposed on top, you might get to the point where signal is DC pulsating between two levels, one of which goes over ADC range. But still might be too fast switching for meter to up range... Autoranging is not magical.. There are scenarios where you can confuse it.

The OP may be able to clear this up, but I didn't see that it was specifically an autoranging issue, more that the meter didn't read OVERLOAD when it was clearly overloaded.

Now the DC vs AC input is an interesting characteristic. There is the external impedance and then there is how things are coupled and measured internally.  For the Fluke 116, for example, VAC and mVAC are both DC-coupled inputs, but internally it appears that VAC is AC-coupled whereas mVAC is not.  So given a 0-1V square wave, the 600mVAC range displays 'OL' but the VAC range correctly displays ~0.500V.  And given a 0-500mV square wave, the 600mVAC range displays ~350mV but the VAC range displays ~0.250VAC.  So mVAC is TRMS AC+DC.  And a closer look at the front panel reveals that I should have known that, but I didn't.   



Now the OPs meter gives the option of DC, AC and DC+AC.  It functions properly on DC and DC+AC, but fails to uprange or indicate overrange on the AC scale, and then only in one direction.  That seems wrong to me.  Thinking about it, perhaps there is some clamping bit that is breaking down too soon and actually keeping the sensed voltage from reaching the overrange threshold.  If some other EEVBlogger has a 786 or 789, perhaps they could help the OP out by running this test and seeing if it is a bug/feature or if the OPs unit has a defect.

OP is pushing signal that is 1 V P-P, with offset such that signal dips only 200mV below zero. He is doing it on a 600mV range.
That range CANNOT range up. It is mV position on the switch. So meter being DC coupled it is overloaded and shows, pretty much crap...
 "......something above 1 V p-p (so above the specs for the mV scale) it reads around
650mv on the AC scale, and it stays so even if I raise the voltage to around 30 V .(Can't get further) ....."

He pushed 30V in 600mV input, that is NOT AC coupled.  So ADC reads full scale and that's it.   As I said there is NO ranging in that switch position. It is fixed 600mV position. I do agree that OVRLD indicator (together with backlight flashing) should be shown instead.

[bdunham7]

That range CANNOT range up... there is NO ranging in that switch position. It is fixed 600mV position. I do agree that OVRLD indicator (together with backlight flashing) should be shown instead.

OK, then it definitely isn't an autoranging issue! 

Yes, the lack of an overrange indicator in one direction seems to me to indicate a specific defect.

OP:  Send your meter back and if you get a replacement, tell us what it does.

Edit:  After thinking further, I'm still not a fan of the meter not reading a 1V_p-p signal in mVAC mode because that does seem to be explicitly within range--0.5VAC.  A true AC mode would imply blocking a reasonable DC component at some point in the signal chain.  That's what my 189 that has all three modes does.

[Neutrion]

But do you have a meter which also has the same AC,  AC+DC, DC range? Because AC+DC is OK and it overranges properly.

I already contacted Brymen through the seller as mentioned, and their ansver was, that this is OK because the meter works well WITHIN the specs. So I suppose all the BM789 should work like this, and possibly the 786 as well. I hope some other guys with 789s will show up.

And the question is, will it be possible at all to change this in FW.

By the way 2N3055 did you check the 869s whether it does the same?

[2N3055]

It doesn't behave the same but it is the same problem:

mV is same, only 600mV (500mV on BM689S) DC range.
For instance, if I feed 300mV RMS 400Hz signal with 700mV offset I get propper 300mV AC reading. If I go higher offset than that, it starts showing bad AC amplitude, less than it is. Until DC offset reaches 8V when it shows overload.
Any do you know why? Because you are (me in this case) severely overloading input amplifier with DC.

If you have mV level AC signal, you need to decouple it from DC by using capacitor as I said. Then you disconnect capacitor and measure DC volts separately.

There is  this misunderstanding that meter can simultaneously measure 10 mV AC  and 40V DC , or vice versa, because it has dual display.
It cannot.  Meter has ONE input attenuator and one ADC.  If you set it for 100V it cannot be in position for 10mV. And vice versa.

Meter that could measure 1 mV AC riding on top of 300V DC would need 2 separate  front end paths, with 2 separate attenuators and front end amplifiers. ADC could be multiplexed, but it would be nice to have two of those too.

Then you could simultaneously measure both AC and DC, completely independently (and at different scales)  at the same time and calculate AC+DC RMS and have that too. You could also measure frequency and duty cycle from AC path with good sensitivity. Also you could optimize AC and DC paths, AC for frequency response, and DC for best AC rejection.

I personally don't know of any meters that do that. I wish it existed, I would gladly buy one.
People that  work on tube amplifiers would love it.

It is up to the operator to know what to do or not.

Good practice requires that you start with meter in higher ranges anyways. Start with measuring AC  and DC in volts range that will also autorange where it needs to be. If there is ANY mention of volts then and there, you CANNOT measure in mV.
If you do have 10mV AC  riding on top of 10V DC, you can use capacitor. That will work well.

If you have 150V AC riding on top of 1 mV DC, you're out of luck. You are asking meter to suppress difference in amplitude of 150000 times...
Funny enough, meter might  have more suppression than that at 50Hz.. But input might start smoking too...
It probably won't, because Brymens have fantastic protection circuits.

But don't you ever confuse what meter can SURVIVE with what is expected for meter to measure.
You can only expect it to measure what is inside range of peak voltage of that range.
Everything else is abuse and survival of user error..

[EEVblog]

And the question is, will it be possible at all to change this in FW.

If it's related to the lack of a DC blocking cap in AC mV mode then there is likely nothing you can in firmware.
Dual display meters have a physical DC measurment path a separate AC coupled path, and they get measured separately.

[2N3055]

And the question is, will it be possible at all to change this in FW.

If it's related to the lack of a DC blocking cap in AC mV mode then there is likely nothing you can in firmware.
Dual display meters have a physical DC measurment path a separate AC coupled path, and they get measured separately.

They still have same input attenuator..

BM869 has DC blocked AC path on AC volts..
But it is not blocked on mV range. Despite having two positions on switch, both AC mV and DC mV is DC coupled.
So injecting 10V DC into it will confuse meter, despite being in AC mV mode.
AC volt ranges are fine.
I don't understand why they didn't DC block it on a dedicate AC mV position either..

On BM78X series they simply dispensed with a confusion and have single mV switch position that is DC coupled.

[bdunham7]

But do you have a meter which also has the same AC,  AC+DC, DC range? Because AC+DC is OK and it overranges properly.

Yes, the Fluke 189 has AC, AC+DC and DC.  It's a little different because the AC range is a separate switch position.

I already contacted Brymen through the seller as mentioned, and their ansver was, that this is OK because the meter works well WITHIN the specs. So I suppose all the BM789 should work like this, and possibly the 786 as well. I hope some other guys with 789s will show up.

Well first of all, if it is overrange, it should say so, not just give you a wrong number.  Second, it happens in one direction, leading me to think there's a specific defect.  Third, what you are asking it to do is not unreasonable and the correct result if it is properly measuring AC of a 0-1V square wave is 500mV--which is within the range.  So why does it read 650mV??

[2N3055]

But do you have a meter which also has the same AC,  AC+DC, DC range? Because AC+DC is OK and it overranges properly.

Yes, the Fluke 189 has AC, AC+DC and DC.  It's a little different because the AC range is a separate switch position.

I already contacted Brymen through the seller as mentioned, and their ansver was, that this is OK because the meter works well WITHIN the specs. So I suppose all the BM789 should work like this, and possibly the 786 as well. I hope some other guys with 789s will show up.

Well first of all, if it is overrange, it should say so, not just give you a wrong number.  Second, it happens in one direction, leading me to think there's a specific defect.  Third, what you are asking it to do is not unreasonable and the correct result if it is properly measuring AC of a 0-1V square wave is 500mV--which is within the range.  So why does it read 650mV??

He was measuring "more than 1V" with excursions below zero of 100-200 mV. That makes it 1.2-1.3V P-P and right at the saturation point.
It had to simply show "OVL" and that is it.

When I do the same to BM869S it will show "OVL". But because of DC path, I can inject DC into it and make it go crazy too, with specific values.
But it behaves much better.

[bdunham7]

For instance, if I feed 300mV RMS 400Hz signal with 700mV offset I get propper 300mV AC reading. If I go higher offset than that, it starts showing bad AC amplitude, less than it is. Until DC offset reaches 8V when it shows overload.
Any do you know why? Because you are (me in this case) severely overloading input amplifier with DC.

That seems to me to be a deficiency in design.  Or at least one not meeting normal expectations.

There is  this misunderstanding that meter can simultaneously measure 10 mV AC  and 40V DC , or vice versa, because it has dual display.
It cannot.  Meter has ONE input attenuator and one ADC.  If you set it for 100V it cannot be in position for 10mV. And vice versa.
I personally don't know of any meters that do that. I wish it existed, I would gladly buy one.
People that  work on tube amplifiers would love it.

The old Fluke 867 Graphing Multimeter could do this, but I can't think of any others.  Other than that, the answer to simultaneous DC and ripple measurement is two meters or a 6.5 digit DMM which can measure 10mV on the 100V scale.  Or you can just switch back and forth.

If you do have 10mV AC  riding on top of 10V DC, you can use capacitor. That will work well.

If you have 150V AC riding on top of 1 mV DC, you're out of luck. You are asking meter to suppress difference in amplitude of 150000 times...
Funny enough, meter might  have more suppression than that at 50Hz.. But input might start smoking too...

There's a difference between normal mode rejection of AC in the DC mode, which seems an unavoidable limitation to me, and blocking of DC in a true AC mode (not AC+DC) which to me seems a normal and expected function of a meter.  I can read the ripple (only) of a PSU by simply selecting mVAC and connecting the leads.  Of course, the F116 fails this test, but it also tells me that it isn't true AC on the front panel and it indicates overload.  I then have the option of moving the selector switch and using the 6V true AC mode, which fully blocks DC.  I would be interested to see what happens on the other ranges of the Brymen because they appear to have adopted the same strategy of using a single switch position for all 3 functions and then selecting the mode with the button.

[Caliaxy]

Well, if this is not a bug and it works as intended, it's a design fail.

I replicated the issue on all the Brymen meters within my reach (235, 789 and 689S): AC mV shows nonsensical values in the presence of a high DC offset. The DC offset doesn't have to be huge. I tried 500mV AC on top of a 2V DC offset - something that one would likely encounter in normal everyday life.

Other cheap(er) meters (Fluke 101 and 107) do the same (show nonsensical values rather than OL). Daves GW121 does it too.

Other even cheaper ones (UT81E and Aneng 8009) don't, they correctly display OL (kudos to them). Fluke 189 (in AC mV) is not tricked by a high DC offset either. Fluke 87V doesn't have this issue at all because it doesn't have a separate AC mV position on the dial in the first place 

I guess it's one of those situations of "know your meter's shortcomings" and "use it accordingly". Check for DC bias before measuring AC mV. No meter is perfect.

Thanks for letting us know!

[2N3055]

The old Fluke 867 Graphing Multimeter could do this, but I can't think of any others.  Other than that, the answer to simultaneous DC and ripple measurement is two meters or a 6.5 digit DMM which can measure 10mV on the 100V scale.  Or you can just switch back and forth.

That is why I usually simply use BM869S in volt mode most of the time. It has enough resolution to show you milivolts even then..
And if I need more detail, I already know what signal looks like and know mV range will give me no problems.

[2N3055]

Well, if this is not a bug and it works as intended, it's a design fail.

I replicated the issue on all the Brymen meters within my reach (235, 789 and 689S): AC mV shows nonsensical values in the presence of a high DC offset. The DC offset doesn't have to be huge. I tried 500mV AC on top of a 2V DC offset - something that one would likely encounter in normal everyday life.

Other cheap(er) meters (Fluke 101 and 107) do the same (show nonsensical values rather than OL). Daves GW121 does it too.

Other even cheaper ones (UT81E and Aneng 8009) don't, they correctly display OL (kudos to them). Fluke 189 (in AC mV) is not tricked by a high DC offset either. Fluke 87V doesn't have this issue at all because it doesn't have a separate AC mV position on the dial in the first place 

I guess it's one of those situations of "know your meter's shortcomings" and "use it accordingly". Check for DC bias before measuring AC mV. No meter is perfect.

Thanks for letting us know!

Are you sure about F87V ? Connect other meter in ohms mode and measure input resistance of F87V. Go through modes and ranges. Not only input resistance won't be constant 10 MOhm, but when it goes Hi impedance then it is AC coupled.

If it is DC coupled on mV range, I bet you it can be made to show crap..
Unless they, for some reason, kept 10 MOhm termination and then AC coupled after that. Which would be good design. I wish Brymen did it that way.

Longer I have MTX3293, better i like it . It has clear AC only (capacitor coupled), AC+DC, DC (both DC coupled, obviously) and Low Z modes right there and single inputs 100-mV-1000V with distinction that 100mV won't autorange.
But BM869S sits right next to it. Fantastic meter, but nothing is perfect.

I have few complaints about MT3293 too:
- Small fonts on big screen
- despite having all the hardware it cannot display AC, AC+DC, DC triple display.

Nothing is perfect.

[bdunham7]

Are you sure about F87V ? Connect other meter in ohms mode and measure input resistance of F87V. Go through modes and ranges. Not only input resistance won't be constant 10 MOhm, but when it goes Hi impedance then it is AC coupled.

If it is DC coupled on mV range, I bet you it can be made to show crap..
Unless they, for some reason, kept 10 MOhm termination and then AC coupled after that. Which would be good design. I wish Brymen did it that way.

I don't have the 87V, but my other models actually do keep the 10M (or a little higher on the low ranges--it's not completely constant) resistance in AC mode.  This is actually an important feature if you use certain HV probes because otherwise if you were measuring HV AC+DC you might accidentally end up with the entire HVDC component across the blocking capacitor.

it cannot display AC, AC+DC, DC triple display.

My F289 also declines to give me all 3 at once and I think it is because they actually don't have the hardware.  I haven't thought about it too hard though....

[Neutrion]

He was measuring "more than 1V" with excursions below zero of 100-200 mV. That makes it 1.2-1.3V P-P and right at the saturation point.
It had to simply show "OVL" and that is it.

When I do the same to BM869S it will show "OVL". But because of DC path, I can inject DC into it and make it go crazy too, with specific values.
But it behaves much better.

Yes that is correct up to the limit it was measuring OK the 650mV came somwhere above the limit.

But does the 869 correctly shows "OL" for AC in this scenario?  Did you try to switch polarity?

With the 869  it makes sense to have everything on the same switch position, but with the single display if there is no way to show overrange they certainly could put in a separate AC mV position.

And the question is, will it be possible at all to change this in FW.

If it's related to the lack of a DC blocking cap in AC mV mode then there is likely nothing you can in firmware.
Dual display meters have a physical DC measurment path a separate AC coupled path, and they get measured separately.

But is also impossible to show somehow that it overranged?

Why is that?

Well, if this is not a bug and it works as intended, it's a design fail.

I replicated the issue on all the Brymen meters within my reach (235, 789 and 689S): AC mV shows nonsensical values in the presence of a high DC offset. The DC offset doesn't have to be huge. I tried 500mV AC on top of a 2V DC offset - something that one would likely encounter in normal everyday life.

Other cheap(er) meters (Fluke 101 and 107) do the same (show nonsensical values rather than OL). Daves GW121 does it too.

Other even cheaper ones (UT81E and Aneng 8009) don't, they correctly display OL (kudos to them). Fluke 189 (in AC mV) is not tricked by a high DC offset either. Fluke 87V doesn't have this issue at all because it doesn't have a separate AC mV position on the dial in the first place 

I guess it's one of those situations of "know your meter's shortcomings" and "use it accordingly". Check for DC bias before measuring AC mV. No meter is perfect.

Thanks for letting us know!

But if the mentioned meters can also display all the different things on the mV scale just like the Brymen, than it should not be fundamentally impossible to show the fact that it overranged. What causes it with the Brymen? The chipset? The input circuit? And as I said it depends on thefrequency.

[Caliaxy]

Other even cheaper ones (UT81E and Aneng 8009) don't, they correctly display OL (kudos to them). Fluke 189 (in AC mV) is not tricked by a high DC offset either. Fluke 87V doesn't have this issue at all because it doesn't have a separate AC mV position on the dial in the first place 

Are you sure about F87V ? Connect other meter in ohms mode and measure input resistance of F87V. Go through modes and ranges. Not only input resistance won't be constant 10 MOhm, but when it goes Hi impedance then it is AC coupled.

I only meant that Fluke 87V doesn't have an "AC mV" mode on the dial at all, only a "DC mV" one (which has a single 600 mV range). It does measures AC mV in "AC V" mode, which goes down to 600 mV range (and all AC ranges are AC coupled). The lowest DC V range in "DC V" mode is 6 V, hence the separate "DC mV" 600 mV DC mode.

Not sure why they did that - maybe it was their way of dodging the bullet. They must have figured the problem and this is how they avoided it. They don't call themselves Fluke for no reason   

[2N3055]

But does the 869 correctly shows "OL" for AC in this scenario?  Did you try to switch polarity?

With the 869  it makes sense to have everything on the same switch position, but with the single display if there is no way to show overrange they certainly could put in a separate AC mV position.

I think we have a language barrier here.

BM869S has 2 different positions for DC and AC mV, but both are nevertheless still DC coupled .

It does show OVL if you apply larger AC voltage. If you combine it with DC, that combined goes over range it also behaves funny.

For instance 1V P-P 400HZ square wave, will show correct 500mV RMS. If I add more than 0.6 V DC to it (offset) it will start showing wrong numbers. Funny enough, I can add -2,3V offset before it starts making problems. So it is not symmetric.

But that is all getting into non important detail. Instrument shouldn't be used outside it's specified range. Good practice is to measure in Volts range all the time and only jump to mV range when we know there won't be hidden problems.

[2N3055]

Other even cheaper ones (UT81E and Aneng 8009) don't, they correctly display OL (kudos to them). Fluke 189 (in AC mV) is not tricked by a high DC offset either. Fluke 87V doesn't have this issue at all because it doesn't have a separate AC mV position on the dial in the first place 

Are you sure about F87V ? Connect other meter in ohms mode and measure input resistance of F87V. Go through modes and ranges. Not only input resistance won't be constant 10 MOhm, but when it goes Hi impedance then it is AC coupled.

I only meant that Fluke 87V doesn't have an "AC mV" mode on the dial at all, only a "DC mV" one (which has a single 600 mV range). It does measures AC mV in "AC V" mode, which goes down to 600 mV range (and all AC ranges are AC coupled). The lowest DC V range in "DC V" mode is 6 V, hence the separate "DC mV" 600 mV DC mode.

Not sure why they did that - maybe it was their way of dodging the bullet. They must have figured the problem and this is how they avoided it. They don't call themselves Fluke for no reason   

Doing AC mV right is not easy. And also, we keep forgetting, as much as people love F87V, it is not a meter for electronics. It is industrial electrician workhorse meter. That is what Fluke says too. For that audience, AC mV is not a thing.

[bdunham7]

he lowest DC V range in "DC V" mode is 6 V, hence the separate "DC mV" 600 mV DC mode.

Not sure why they did that

It would be interesting to see what its capability for normal mode AC rejection is in that mVDC range.

[Caliaxy]

Other even cheaper ones (UT81E and Aneng 8009) don't, they correctly display OL (kudos to them). Fluke 189 (in AC mV) is not tricked by a high DC offset either. Fluke 87V doesn't have this issue at all because it doesn't have a separate AC mV position on the dial in the first place 

I'll have to take back the statement that UT81E doesn't have this issue. I just realized that 500 mVrms in my quick test was way over its AC mV range (200 mV or so), which might have contributed to its displaying OL. At 100 mVrms it also displays nonsense at DC offsets higher than 2V (but it seems a bit more tolerant to DC bias than the Brymens). Aneng 8009 might behave similarly (haven't rechecked, but its upper AC mV limit is only 100 mV or so...).

Fluke 189 works fine (and so does Keysight U1282A, if anyone wonders; U1252A doesn't).

[bdunham7]

Well, I guess part of the answer is "RTFM"!  At least they've documented this issue.  I still think the OP may have an issue though, as I think it should indicate OL, but I'll await someone else chiming in with some tests.  The F116 and F189 don't have this limitation--I tried a 0-2V 5% duty cycle signal (CF~5) and they both showed approximately the expected readings. 

[Neutrion]

I think we have a language barrier here.

BM869S has 2 different positions for DC and AC mV, but both are nevertheless still DC coupled .

It does show OVL if you apply larger AC voltage. If you combine it with DC, that combined goes over range it also behaves funny.

For instance 1V P-P 400HZ square wave, will show correct 500mV RMS. If I add more than 0.6 V DC to it (offset) it will start showing wrong numbers. Funny enough, I can add -2,3V offset before it starts making problems. So it is not symmetric.

But that is all getting into non important detail. Instrument shouldn't be used outside it's specified range. Good practice is to measure in Volts range all the time and only jump to mV range when we know there won't be hidden problems.

So if I understand you right, the 869s also does this or something similar? What does it show on the AC mV range if you feed in a similar 30V p-p squarewave?

Again I don't want to use the meter above its specified range (1 V p-p offset included) all what I would like, that the meter shows me that it overranged. Like it shows it correctly on DC mV or AC+DC mV.

Othervise when one measures around 5-600mV AC(correctly, because that is still within its range), and something goes wrong, and let's say 230V appears, but one won't have any clue about that. Because the meter shows almost exactly the same values.
But even a 30 V extra can be quiet problematic of course.

My question about the meter was also about the possibilities of showing "OL" above the 1V p-p level, and not whether it would be possible to measure mV AC on top of some dc offset above the specified 1V p-p treshold.

[2N3055]

I think we have a language barrier here.

BM869S has 2 different positions for DC and AC mV, but both are nevertheless still DC coupled .

It does show OVL if you apply larger AC voltage. If you combine it with DC, that combined goes over range it also behaves funny.

For instance 1V P-P 400HZ square wave, will show correct 500mV RMS. If I add more than 0.6 V DC to it (offset) it will start showing wrong numbers. Funny enough, I can add -2,3V offset before it starts making problems. So it is not symmetric.

But that is all getting into non important detail. Instrument shouldn't be used outside it's specified range. Good practice is to measure in Volts range all the time and only jump to mV range when we know there won't be hidden problems.

So if I understand you right, the 869s also does this or something similar? What does it show on the AC mV range if you feed in a similar 30V p-p squarewave?

Again I don't want to use the meter above its specified range (1 V p-p offset included) all what I would like, that the meter shows me that it overranged. Like it shows it correctly on DC mV or AC+DC mV.

Othervise when one measures around 5-600mV AC(correctly, because that is still within its range), and something goes wrong, and let's say 230V appears, but one won't have any clue about that. Because the meter shows almost exactly the same values.
But even a 30 V extra can be quiet problematic of course.

My question about the meter was also about the possibilities of showing "OL" above the 1V p-p level, and not whether it would be possible to measure mV AC on top of some dc offset above the specified 1V p-p treshold.

Problem is that by pushing 30V in mV range meter circuitry is overloaded in such a way that not even OVL detection works anymore.
OVL is simply coming from ADC overrange register. If preamps are overloaded in such a way that that they are producing something inside ADC input range, meter (ADC) "doesn't see" overrange. That is simple fact based on fact that you need amplifier in front of ADC for sensitive range.

To circumvent around that, a separate, parallel circuitry could be added that would supervise input and monitor for overload. That circuit could be even used for autoranging.. But that would just add to the price. It is responsibility of the user not to connect meter to 10000V in millivolt mode.
It is your responsibility to know the 230V cannot appear from nowhere. Not only because of meter (it will survive) but because of you. You have to have clue. Or big badaboom...

You should be using old moving coil meter for 6 months and you would understand me well.
On those, OVL indicator was smoke coming from it...

Don't get me wrong, it would be nice if they could make completely foolproof meter.. But with good measurement practice that is not necessary.
Also good measurement practice takes into account your safety. Which always comes first.

[bdunham7]

OVL is simply coming from ADC overrange register. If preamps are overloaded in such a way that that they are producing something inside ADC input range, meter (ADC) "doesn't see" overrange. That is simple fact based on fact that you need amplifier in front of ADC for sensitive range.

To circumvent around that, a separate, parallel circuitry could be added that would supervise input and monitor for overload. That circuit could be even used for autoranging.. But that would just add to the price. It is responsibility of the user not to connect meter to 10000V in millivolt mode.

I think some systems do use as separate path for the autorange to make it faster, but it is filtered.  You'd need to tap into it earlier for the overload indicator.  Some meters are better than others at dealing with this--try a 50Hz 1% duty cycle 0-5V square wave if you want a brutal test.  (CF = 10) Even my 8846A didn't range correctly on AC with that one.  However, the issue with the OPs example is that it seems to be a pretty basic example that I wouldn't expect it to not perform well at--but there it is in the specs.  A crest factor of only 1.6 might not be enough to measure some modified sine wave inverter outputs or triac circuits, among other things.

[joeqsmith]

1.If I use the mV Scale and feed in a squarewave with 50% duty cycle without(or very minimal like 100-200mV) zero crossing ...

I started to read your original post but the was not sure with all the missing details.  mV (rms, p-p, average, DC).  I wonder is scale DC, AC, AC+DC.   Want me to try and guess?    What does zero crossing mean to you?  Maybe use terms like offset.   

I have no problems running what ever test cases you want but you need to be VERY clear about what you want to see.   Even a scope shot would help.   


****
So, the BM789 is set to ACmV autorange.  You inject a 40Hz to 500Hz, 50% duty squarewave with no offset and 100mVp-p (+/-50mV) and the meter reads fine.  You then increase the amplitude to 1Vp-p and your meter displays 650mV.  You then continue to drive it up to 30Vp-p and the meter continues to display 650mV.   Then you flip the leads (even though there is no offset and they waveform is symmetrical) and the meter starts to read correctly.   You have verified the wavefrom you are injecting by other means (scope) and know it to be symmetrical.   

Then you go onto to talk about  " I assume it depends on which polarity the squarevawe has."  which now I am thinking it is not a symmetrical waveform but rather switching from 0 to some peak value.       Then later you mention  " (1 V p-p offset included) "   

I thought if I waited and reread your post I could make sense of it.   My guess is you are adding an offset in which case, I bet you could make the meter do all sorts of strange things and I can believe that flipping the leads would make a difference.   A bit of an odd use case but I won't judge. 

Maybe come at it from another direction and post your actual requirements.  It seems you can't use a blocking cap which is typical and you don't want to have to get into the practice of starting in the high ranges.   So, I would say it sounds like you bought the wrong tool for the job. 
So maybe start with the requirements and someone could maybe recommend something. 
 

[Neutrion]

You should be using old moving coil meter for 6 months and you would understand me well.
On those, OVL indicator was smoke coming from it...

Don't get me wrong, it would be nice if they could make completely foolproof meter.. But with good measurement practice that is not necessary.
Also good measurement practice takes into account your safety. Which always comes first.

Obviously I don't use the meter with starting on the millivolt scale, but things can happen in life (at least with me) which we don't foresee so I would be rather on the safe side when measuring something unknown, and be sure that the values around and above 500mV AC are also corect, and I don't have to worry that the reality is something entirely different. With a dual display, as I said it might won't be an issue as you could see on the DC or AC+DC value, that you are out of range but with the single display this might pose a problem.  So this is just safety, like the fuses in your meter, which are also not supposed to be blown- at least regulary.

Joe:

Without zero crossing I meant an offset, so it is an assymetrical squarewave. Meter scale is at AC mV.

An example:Assymetrical squarewave duty cycle 50% p-p 5,28V average 2,52V max 5,12V min -160mV rms 3,55 V
AC V scale correct display: 2,5-2,7 V(STdev on Siglent) which is correct, mV scale: 650mV
Will add a scopeshot if needed later on.
At 1V p-p the meter measures about correctly(not 650mV) so I am not talking about it not meeting the specs, things start to happen if I go above the limit.

I don't have a signal gen, so I can't just test it with different offset levels. But within the 1V p-p including offset there seems to be no problem with the measurement. So the specifications are seemingly met.
I can not tell you what it does if I feed in the same signal without offset and raise it above the 1V p-p level, but it seems, that at least if I go above the specified limit with an assymetrical signal (which again I in a normal case obviously don't want to do, but could happen during a measurement) the AC mV scale does not show that it overranged, but stays like that at around 650mV and changes only minimally.
If I flip the leads it shows "OL" what should be the correct behaviour.
So with changed polarity I meant that the sqarewave is assymetrical signal starts almost at zero, and goes up to above 1V, positive, so the meter gets the signal with a given polarity, but if I flip the leads the meter gets the signal with the opposite polarity.

[joeqsmith]

Now that it is clear you are adding a DC offset,  none of what you wrote surprises me.     At first I thought it was the wrong tool for the job but not posting your requirements, it seems you're just playing around with your new toy.   Most of the responses here seem correct. 

[Fungus]

AC V scale correct display: 2,5-2,7 V(STdev on Siglent) which is correct, mV scale: 650mV

When I teach my basic multimeter class I always tell them to use the 10A input jack for measuring current and only move to the mA input jack if they've seen a signal too small to measure properly on the 10A scale.

The same reasoning applies here too. mV/mA ranges are designed for sensitivity, not to handle overloads.

Now that it is clear you are adding a DC offset,  none of what you wrote surprises me.

You've done several similar tests on bigger scale, adding hundreds of volts of offset on the ACV scale. Many meters fail to handle it gracefully.

it seems you're just playing around with your new toy.

Which is fine! It's good to know the limitations of the tools we own, playing is a good way to find them.

[joeqsmith]

Now that it is clear you are adding a DC offset,  none of what you wrote surprises me.

You've done several similar tests on bigger scale, adding hundreds of volts of offset on the ACV scale. Many meters fail to handle it gracefully.

I certainly have and in cases where the meter doesn't want to switch to the next range,  it makes for a less than impressive review.   I was glad Brymen's engineers were able to sort it out before the meters release.   

The BM789 is not a bad little meter.  Add a few changes like split display and I think I would have a new favorite.    Considering the abuse that both BM869s have seen and their continued trouble free service,  its proving to be more difficult to find something I would take over them.

[Neutrion]

Now that it is clear you are adding a DC offset,  none of what you wrote surprises me.     At first I thought it was the wrong tool for the job but not posting your requirements, it seems you're just playing around with your new toy.   Most of the responses here seem correct.

So do you agree with the others, that there is no way of showing the overload under these circumstances?
Does this happen with every major brand with similar funcions?
Playing: Yes, it is not smart to start using a meter or tool if you are not avare of its limitation, or functions so I am just playing. But watched you playing with the 786 before I bought it
Thats why I was curious of your oppinion.

AC V scale correct display: 2,5-2,7 V(STdev on Siglent) which is correct, mV scale: 650mV

When I teach my basic multimeter class I always tell them to use the 10A input jack for measuring current and only move to the mA input jack if they've seen a signal too small to measure properly on the 10A scale.

The same reasoning applies here too. mV/mA ranges are designed for sensitivity, not to handle overloads.

I also do that with the 10A scale, or actually using a clamp meter if possible, and oviously generally not starting to use the meter on the millivolt scale but again, the problem is that measured values can change while one measures them, and even values in the mV range could suddenly change even dramatically, even if they are not supposed to. So I am only missing the overrange indication and asking whether it would really be impossible with the FW update to solve this, or it would require a major HW update.

But I start to have a feeling that what I wish for is something extraordinary .
That is why I was curious how other meters (with a mV scale of course!) would perform here.

If we are talking about best practice, with the 789 it seems, that if one reads anything on the mV AC range higher than 500mV one should not trust those numbers, and constantly doublecheck it on an other scale, or in the dc range as well. Just as a sidenote. If no FW update is possible for this.

[Neutrion]

I certainly have and in cases where the meter doesn't want to switch to the next range,  it makes for a less than impressive review.   I was glad Brymen's engineers were able to sort it out before the meters release.   

The BM789 is not a bad little meter.  Add a few changes like split display and I think I would have a new favorite.    Considering the abuse that both BM869s have seen and their continued trouble free service,  its proving to be more difficult to find something I would take over them.

This missing split display was for me the only major point why I could not decide for a while between the 789 and the 869. Is there any major technical reason they decided not to go that way?
Or is it that in some industrial application maybe the big numbers are the number one priority?
I also really like the warm white backlight, which is uniqe.

[bdunham7]

mV/mA ranges are designed for sensitivity, not to handle overloads.

Typical designs of AC/mVAC ranges include blocking capacitors at some point to prevent any overload by a DC bias.  This is how you would read ripple on a power supply, right?  This meter is atypical, although documented in the specs, and would be unable to read power supply ripple in the mVAC range without an external blocking capacitor.  This may not be a deal killer for most users, but it is a shortcoming that cannot be overcome with 'correct technique'.  You have to add an external component or move up to a higher range and lose resolution.  So now we know.

You've done several similar tests on bigger scale, adding hundreds of volts of offset on the ACV scale. Many meters fail to handle it gracefully.

That was simply a ranging issue.  The vast majority of meters will not be able to handle over-range normal mode AC rejection, it's just an inherent limitation that can't be easily solved with one component.  The Brymens can't handle over-range normal mode rejection either, but the autoranging appears to respond to the AC enough to move up to a higher range, at least under those test conditions.  That's an advantage of certain types of fast autoranging circuits. 

[AndrewBCN]

...
Which is fine! It's good to know the limitations of the tools we own, playing is a good way to find them.

Except in this case it's not exactly a "limitation" of the BM789. The OP stumbled upon an edge case which is very similar to an operator error, where the BM789's autorange algorithm doesn't compute an appropriate response. The obvious "fix" for this issue is not to reprogram the MCU inside the multimeter (because there is an almost unlimited number of similar edge cases), but to operate the Brymen BM789 following standard good practices, which even a beginner can learn is less than 15 minutes.

Also note that the edge case / wrong operation of the BM789 that the OP stumbled upon is completely different from the methodical stress testing that joeqsmith performs on his DMMs. Joe simulates real-life accidents and situations that the meters are supposed to be able to cope with, within and sometimes beyond their specifications.

Joe stresses the hardware and points to flaws in the design and safety of DMMs, the OP just found an operator error that the programmers at Brymen didn't account for in their autorange algorithm.

I very much enjoy Joe's videos where he goes through various steps to test his DMMs, because what I see is a talented engineer making sure his tools are fit for the job, and this information is very valuable. An operator error that leads to a DMM incorrect display is anecdotal information of next to zero value.

[Fungus]

I also really like the warm white backlight, which is uniqe.

OT, but... Brymen do some orange backlights, too, eg. my BM857. 

[bdunham7]

So do you agree with the others, that there is no way of showing the overload under these circumstances?
Does this happen with every major brand with similar funcions?

To be clear, since you are implying what 'others' agree on and I'm an other, I don't agree and think the meter should show an overload indication.  Whether that's a design issue or a one-off defect with your unit, I don't know.

And no, this doesn't happen with most meter designs AFAIK, at least not the ones I have with an explicit mVAC function.  I think it is a result of 'bangforbuckitis' or the manufacturer trying to add as many features as possible at a price point.  Something has to give.

If we are talking about best practice, with the 789 it seems, that if one reads anything on the mV AC range higher than 500mV one should not trust those numbers, and constantly doublecheck it on an other scale, or in the dc range as well. Just as a sidenote. If no FW update is possible for this.

This shouldn't be necessarily be dealbreaker, its just another characteristic you have to learn to deal with--if your individual meter is not, in fact, defective.  I think the easiest way to guard for it would be to always take your reading initially  in mVAC+DC, then if it reads in range you can switch to mVAC and get the correct reading.  This actually isn't a bad idea for any AC range on that meter, since it is easy to do with just two button pushes.

And, b/t/w, contrary to some earlier discussion, if this behavior is characteristic of all the units, I think issue of the reading hanging without an OL indication might well be resolvable in firmware. 

[joeqsmith]

Now that it is clear you are adding a DC offset,  none of what you wrote surprises me.     At first I thought it was the wrong tool for the job but not posting your requirements, it seems you're just playing around with your new toy.   Most of the responses here seem correct.

So do you agree with the others, that there is no way of showing the overload under these circumstances?

I believe that given a set of requirements, a good group of EEs could design products to achieve them. 

Does this happen with every major brand with similar funcions?

As I already mentioned, its not how I would normally use a meter.   Another good example of what I would consider a fringe case, someone had posted how they were trying to measure a high value resistor that was right on the edge of the range switch point.  The meter was picking of the AC mains and had problems.  The tried a similar test using other meters and they did not have the problem.  Something like that.      In this case, I had the same meter and I think I was able to replicate it by twisting the probes with a lamp cord while trying to make the measurement.  I think in that case, the company came up with a hardware solution but it's nothing I considered adding as I am not normally wrapping my leads around lamp cords.   

[Fungus]

This meter is atypical, although documented in the specs, and would be unable to read power supply ripple in the mVAC range without an external blocking capacitor.

Yes, but it's a 50,000 count meter so you should be able to read power supply ripple perfectly well on the normal ACV range.

The only problem is that you have to read the fine manual.

You've done several similar tests on bigger scale, adding hundreds of volts of offset on the ACV scale. Many meters fail to handle it gracefully.

That was simply a ranging issue.

I wasn't referring to that specific case of the 87V giving incorrect readings.

The vast majority of meters will not be able to handle over-range normal mode AC rejection, it's just an inherent limitation that can't be easily solved with one component.

Yep. Joe gave the 87V a pass on the incorrect reading because it's quite atypical.

[bdunham7]

Except in this case it's not exactly a "limitation" of the BM789.
An operator error that leads to a DMM incorrect display is anecdotal information of next to zero value.

The meter is unable to read AC ripple on a DC bias in the mVAC range, such as a very common PSU ripple measurement.  How is that not a limitation?

As for the value of the information, I don't know why you think it has zero value.  It certainly is instructive about how to avoid a particular issue in using the meter.  Of course, as I posted, it's in the manual too, but who reads those?

[Fungus]

To be clear, since you are implying what 'others' agree on and I'm an other, I don't agree and think the meter should show an overload indication.  Whether that's a design issue or a one-off defect with your unit, I don't know.

That assume the meter can detect the condition, which is an unknown.

The way to find out would be to have Joe do a video on it. If Brymen fixes it and sends him another meter for his collection then we know it's a firmware issue.   

And no, this doesn't happen with most meter designs AFAIK, at least not the ones I have with an explicit mVAC function.  I think it is a result of 'bangforbuckitis' or the manufacturer trying to add as many features as possible at a price point.  Something has to give.

Yep. This is a massive bangperbuck meter. Maybe that capacitor plus a few other components was just a bit too much.

[Neutrion]

...
Which is fine! It's good to know the limitations of the tools we own, playing is a good way to find them.

Except in this case it's not exactly a "limitation" of the BM789. The OP stumbled upon an edge case which is very similar to a operator error, where the BM789's autorange algorithm doesn't compute an appropriate response. The obvious "fix" for this issue is not to reprogram the MCU inside the multimeter (because there is an almost unlimited number of similar edge cases), but to operate the Brymen BM789 following standard good practices, which even a beginner can learn is less than 15 minutes.

Also note that the edge case / wrong operation of the BM789 that the OP stumbled upon is completely different from the methodical stress testing that joeqsmith performs on his DMMs. Joe simulates real-life accidents and situations that the meters are supposed to be able to cope with, within and sometimes beyond their specifications.

Joe stresses the hardware and points to flaws in the design and safety of DMMs, the OP just found an operator error that the programmers at Brymen didn't account for in their autorange algorithm.

I very much enjoy Joe's videos where he goes through various steps to test his DMMs, because what I see is a talented engineer making sure his tools are fit for the job, and this information is very valuable. An operator error that leads to a DMM incorrect display is anecdotal information of next to zero value.

Sudden unforeseen  levels of voltages are also real life issues, which you don't want, but it just happens.

Now with this meter if you are measuring 300 mV AC, and the value goes over 500mV, you have to doublecheck it on an other scale.

But we are arguing here about whether it is the intended use of the meter: No it is not (except if one wanted to buy the meter for the mentioned ripple measurement.)
But the real question in the whole topic is whether the missing "OL" indication is solveable in FW.
According to Dave possibly not, 2N3055 provided a possible technical description why not, Joe seems to agree?.

I also really like the warm white backlight, which is uniqe.

OT, but... Brymen do some orange backlights, too, eg. my BM857. 

 I also like the orange more than the cool colors. Especially during the winter season it is more confy. But still the warm white is my favourite.
Just waiting to see a user configurable rgb backlight  Deep purple would be also nice before going to bed and taking a final look at the meter before sleeping.

[joeqsmith]

I certainly have and in cases where the meter doesn't want to switch to the next range,  it makes for a less than impressive review.   I was glad Brymen's engineers were able to sort it out before the meters release.   

The BM789 is not a bad little meter.  Add a few changes like split display and I think I would have a new favorite.    Considering the abuse that both BM869s have seen and their continued trouble free service,  its proving to be more difficult to find something I would take over them.

This missing split display was for me the only major point why I could not decide for a while between the 789 and the 869. Is there any major technical reason they decided not to go that way?
Or is it that in some industrial application maybe the big numbers are the number one priority?
I also really like the warm white backlight, which is uniqe.

Outside of any influence my small YT channel and posts on DMMs may have had on peoples purchasing decisions, I am not involved with marketing or sales.    I don't work for Brymen and really have no understanding of the hows and whys they or any other DMM company make the choices they do. 

Again,  I find meters that can display more than one parameter at a time allow me to work more efficient.   That's my personal choice.   As a consumer, it's good to have such choices to make! 

[Fungus]

To be clear, since you are implying what 'others' agree on and I'm an other, I don't agree and think the meter should show an overload indication.  Whether that's a design issue or a one-off defect with your unit, I don't know.

That assume the meter can detect the condition, which is an unknown.

Although... thinking about it... if it's displaying the number "650" on a 60,000 count meter then the ADC is probably seeing binary 11111111111111111111...111111 (however many bits there are in the ADC).

In that case it's firmware fixable. Over to Joe. 

Yep. This is a massive bangperbuck meter.

The hardware limitation could be as simple as them running out of inputs on the ADC's input multiplexer. A separate, AC-coupled circuit would need another input. It might not be possible to do it even though capacitors are cheap.

[joeqsmith]

The way to find out would be to have Joe do a video on it. If Brymen fixes it and sends him another meter for his collection then we know it's a firmware issue.   

  
There was another concern I had early on with the 78x and when I bought it up, the user had not read the manual.        

Adding a capacitor in series sounds like a simple fix but is it really that simple or would it make the meter less robust? Does the entire circuit get more complicated and need many more components than just a capacitor?

I think my old analog meter from my youth had a capacitor built-in with a separate jack similar to that cheapo unit I looked at.  Note that the jack is marked output but you can clearly see the DC blocking cap in the schematic and we know what it's purpose is.   

https://youtu.be/_c7tD7UeXUg?t=1620

I've used DMMs to look at ripple but just add a blocking cap out of habit.  Not a lot of drama.

[Neutrion]

Now that it is clear you are adding a DC offset,  none of what you wrote surprises me.     At first I thought it was the wrong tool for the job but not posting your requirements, it seems you're just playing around with your new toy.   Most of the responses here seem correct.

So do you agree with the others, that there is no way of showing the overload under these circumstances?

I believe that given a set of requirements, a good group of EEs could design products to achieve them. 

Ok I mean in this case with this meter, without(complicated, or "robustness lowering") HW mod.

But it is also interesting what are the general reqirements here.

To be clear, since you are implying what 'others' agree on and I'm an other, I don't agree and think the meter should show an overload indication.  Whether that's a design issue or a one-off defect with your unit, I don't know.

And no, this doesn't happen with most meter designs AFAIK, at least not the ones I have with an explicit mVAC function.  I think it is a result of 'bangforbuckitis' or the manufacturer trying to add as many features as possible at a price point.  Something has to give.

And, b/t/w, contrary to some earlier discussion, if this behavior is characteristic of all the units, I think issue of the reading hanging without an OL indication might well be resolvable in firmware. 

OK sorry, it is also good to know that I might not be alone and a complete idiot with my "requirement".

And you also think it is solvable in FW. Which would be nice.

Outside of any influence my small YT channel and posts on DMMs may have had on peoples purchasing decisions, I am not involved with marketing or sales.    I don't work for Brymen and really have no understanding of the hows and whys they or any other DMM company make the choices they do. 

Again,  I find meters that can display more than one parameter at a time allow me to work more efficient.   That's my personal choice.   As a consumer, it's good to have such choices to make! 

I thought you might could have a general idea -not necesseraly based on hard facts, so the category of oppinion which you seemingly don't like to have:) -  what kind of technical aspect could lay in the background for Brymen choosing the single display.
More processor needed, more complex circuitry other interesting stuff.
I mean generally only complexity and the mentioned possible industrial case speaks against the multi display.Maybe even you could like the meter more than the 869 as you mentioned.

[bdunham7]

Yes, but it's a 50,000 count meter so you should be able to read power supply ripple perfectly well on the normal ACV range.

50k counts helps, but you still have TRMS residual counts and residual count suppression to deal with, and it also depends on how low you want to measure the ripple.  If your spec is 5mV on a 20V supply and you have to read that on the 50V range...

Adding a capacitor in series sounds like a simple fix but is it really that simple or would it make the meter less robust? Does the entire circuit get more complicated and need many more components than just a capacitor?

On the designs I've looked at, adding it early on is fairly simple.  Simply switching it in ahead of everything, giving the meter an infinite DC impedance, requires a HV capacitor but causes no issues.  After the voltage divider/gain stage is a bit trickier.  If you look at your 89IV,  you'll see that it has distinct switched mVAC and VAC ranges and those have the capacitor in front and the DC impedance of the input will be infinite-ish.  Thus you can read microvolts AC on top of a kilovolt DC bias.  The mVDC and VDC ranges, OTOH, have all the AC/AC+DC/DC functions like the BM789 we are discussing and those ranges will have some definite limitations, although I don't know what they are.  The BM789 just omits the switched AC ranges entirely.

It is possible to add the capacitor after the voltage divider, but if you have an early gain stage like you would in a mV range, it becomes difficult to put it anywhere but right in front.  This is why the F116 is able to have a 10M-ish input impedance on all ranges AC and DC, but the mV range actually is only mVAC+DC or mVDC.

Yep. Joe gave the 87V a pass on the incorrect reading because it's quite atypical.

I don't know how atypical--I have an occasional need to measure and set a <5mV DC bias with 30VAC on top of it.  The reason I would give it a pass is because I pretty much give any meter a pass on autoranging issues.  Autoranging has so many compromises that need to be made that I don't think there ever is one 'right' solution.  My 8846A, for example--great meter in most ways, but the autoranging is so twitchy it is laughable.  Unless you actually need it to work, then it isn't funny, it's just utter crap.  On my old 8505A, OTOH, the autoranging is glacially slow, but as far as I can tell is never wrong.

[Fungus]

And you also think it is solvable in FW. Which would be nice.

If might not be as simple as, "add another 'if () ...' statement", in the code editor. 

All multimeter chipsets have very limited space for firmware and most of them end up at 100% usage one way or another.

It might even be a thing they took out to make space for something else.

[joeqsmith]

Sudden unforeseen  levels of voltages are also real life issues, which you don't want, but it just happens.

 
Don't I know it.  Again, it's why I started making reviews showing something besides unboxing them, talking about my years as being a expert in flipping burgers to validate my opinions and then giving them all five stars.   I'm looking for products that survive, not that can make an accurate measurement while being exposed to these various tests.   

But we are arguing here about whether it is the intended use of the meter: No it is not (except if one wanted to buy the meter for the mentioned ripple measurement.)
But the real question in the whole topic is whether the missing "OL" indication is solveable in FW.
According to Dave possibly not, 2N3055 provided a possible technical description why not, Joe seems to agree?.

Sorry, I wasn't aware we were arguing.   I would have no way to know if ANY meter would show over range under all conditions when set to their ACmV function.  Again, it's not something I would normally do and if people agree with me or not is of little concern.   

My collection of DMMs is mostly limited to ones that survived my tests.    Others may offer a broader spectrum to avoid adding bias.  I have little doubt if anyone wanted to try it we would see some problems.   I doubt you would need to run them up to the 30V you mention. 

[AndrewBCN]

The meter is unable to read AC ripple on a DC bias in the mVAC range, such as a very common PSU ripple measurement.  How is that not a limitation?

As for the value of the information, I don't know why you think it has zero value.  It certainly is instructive about how to avoid a particular issue in using the meter.  Of course, as I posted, it's in the manual too, but who reads those?

Personally I don't think a multimeter is the best tool to observe and measure PSU ripple and noise (and transients), I believe a DSO provides a lot more information.

In any case, as you noted, it's right there in the BM789 manual: in the ACmV (600mV) and AC+DCmV ranges, peak value including DC offset must be under 1000mV.

So perhaps people who are trying to find a fault in a multimeter should read the manual first ?
 

[joeqsmith]

As I understand it, for the OP it all comes down to if the over range indicator works under ALL conditions with the meters set specifically to their ACmV function which may not always be found in the manual.  I think this is why they are suggesting someone try running a test.   Again, for me the outcome isn't a big deal one way or the other.  I can believe we would find some cases. 

The problem I see when looking for these fringe cases is the OP has not provided any constraints.  I suspect it's more their general curiosity than anything.   Maybe limit it to the 40-500Hz they mentioned and keep it within +/-2 Volts so everyone could join in the fun? 

[AndrewBCN]

I have one suggestion for the firmware engineers at Brymen: please program the BM789's MCU to display the following characters when the ACmV or AC+DCmV ranges are selected and an input peak voltage including DC offset > 1000mV is detected:

RTFM

Honestly, why didn't they think of that? 

[bdunham7]

Personally I don't think a multimeter is the best tool to observe and measure PSU ripple and noise (and transients), I believe a DSO provides a lot more information.

Clearly this DMM is not the best tool for that, but there are many others that do just fine.  Whether a DSO is necessary or better depends on what you are doing.  What about a quick go/no-go test when a DSO isn't handy?  What if the power supply ripple is below the noise floor of the DSO?  For me personally, this would be a routine test and I would  typically only break out the scope if my DMM indicates a ripple issue or there is some other strange problem.

So perhaps people who are trying to find a fault in a multimeter should read the manual first ?

A documented limitation is still a limitation. 

[joeqsmith]

I think the OPs concern is that meters may read what appears to be a normal level.   They just want to know that they are out of range without using the ACV or any other mode.  Again, just my understanding as I boil these few pages down. 

I can believe we could find meters that would show 0.000 mVAC with volts applied.   

[joeqsmith]

So perhaps people who are trying to find a fault in a multimeter should read the manual first ?

A documented limitation is still a limitation.

It seems like its an undocumented limitation we are after.   

[bdunham7]

The problem I see when looking for these fringe cases is the OP has not provided any constraints. 

I agree, and didn't realize until he posted it that he doesn't have an actual AWG. That makes his test hard to replicate reliably.  I'm sure either you or I can make any meter display a "wrong" reading fairly easily.  A good challenge might be to come up with a signal that 'good' meters get wrong but the Harbor Freight free one reads correctly.

[bdunham7]

It seems like its an undocumented limitation we are after.

Yes, the lack of an overload indication even at higher voltages seems odd, which is why I had speculated that perhaps the clamping circuit was breaking down too early.  But that's just rampant speculation that needs to be tested.

[Caliaxy]

I have one suggestion for the firmware engineers at Brymen: please program the BM789's MCU to display the following characters when the ACmV or AC+DCmV ranges are selected and an input peak voltage including DC offset > 1000mV is detected:

RTFM

Honestly, why didn't they think of that? 

If I understand correctly, the chief complain here is not that the meter cannot measure outside its published specs (DC offset > 1V in AC mV mode), but that it displays erroneous values when this happens (as opposed to "OL"), tricking the unsuspecting user (other people, of course, not us, because now we know it).

BTW, the AC+DC mV mode (a press of a button away from AC mV) displays OL correctly under these conditions. Then you need to press the yellow button 5 more times to return to the AC mV mode (which is why I'm not in love with Brymen...)

[joeqsmith]

A good challenge might be to come up with a signal that 'good' meters get wrong but the Harbor Freight free one reads correctly.

If you want to play around with it a bit and propose something, I would gladly attempt to follow along.   I think the simpler your setup, the better.   Using an Arb seems reasonable and allows automation.   If you have a way to make a video of it all running, even better.   

If I understand correctly, the chief complain here is not that the meter cannot measure outside its published specs (DC offset > 1V in AC mV mode), but that it displays erroneous values when this happens (as opposed to "OL"), tricking the unsuspecting user (other people, of course, not us, because now we know it).

I believe this is what the OP is after.

Don't get me wrong.  I make some pretty boneheaded mistakes but I just don't want the meter to be damaged as a result.

[Neutrion]

I have one suggestion for the firmware engineers at Brymen: please program the BM789's MCU to display the following characters when the ACmV or AC+DCmV ranges are selected and an input peak voltage including DC offset > 1000mV is detected:

RTFM

Honestly, why didn't they think of that? 

RTFM is also an option, but I still vote for OL.

I think the OPs concern is that meters may read what appears to be a normal level.   They just want to know that they are out of range without using the ACV or any other mode.  Again, just my understanding as I boil these few pages down. 

Exactly. And you can say "he". Even without any accurate measurement.

[AndrewBCN]

I have one suggestion for the firmware engineers at Brymen: please program the BM789's MCU to display the following characters when the ACmV or AC+DCmV ranges are selected and an input peak voltage including DC offset > 1000mV is detected:

RTFM

Honestly, why didn't they think of that? 

If I understand correctly, the chief complain here is not that the meter cannot measure outside its published specs (DC offset > 1V in AC mV mode), but that it displays erroneous values when this happens (as opposed to "OL"), tricking the unsuspecting user (other people, of course, not us, because now we know it).
...

And again, perhaps the unsuspecting user should read the fine manual, which I did even though I don't own a BM789, and it took all of 45 seconds to find the specs.
Here: http://www.brymen.com/images/ProductsList/BM780_List/BM789-5-manual-print1-r7.pdf

Pages 26 and 27, if you can only spend 15 seconds to look for the ACmV and AC+DCmV ranges specs.

...
A documented limitation is still a limitation. 

You can call it a documented limitation if you want, but - as you yourself noted - the BM789 ACmV range is DC coupled and even though I have an extremely limited understanding of DMM circuitry, even I can understand that this range was *deliberately* not designed to measure ripple and noise from a PSU. Of course I am assuming that Brymen engineers know what they are doing, but that seems like a reasonable assumption.

[bdunham7]

You can call it a documented limitation if you want, but - as you yourself noted - the BM789 ACmV range is DC coupled and even though I have an extremely limited understanding of DMM circuitry, even I can understand that this range was *deliberately* not designed to measure ripple and noise from a PSU. Of course I am assuming that Brymen engineers know what they are doing, but that seems like a reasonable assumption.

I'm not sure if we have a linguistic hangup here over the term 'limitation' or what, but I don't see the dispute.  If they deliberately design it knowing that it can't do something as a result, then that is a limitation.  If they put it in the manual, then it is documented.  Neither of these terms is inherently pejorative, simply descriptive.  Whether you consider the characteristic (limitation) itself to be a bad thing is up to you.

[Neutrion]

And again, perhaps the unsuspecting user should read the fine manual, which I did even though I don't own a BM789, and it took all of 45 seconds to find the specs.
Here: http://www.brymen.com/images/ProductsList/BM780_List/BM789-5-manual-print1-r7.pdf

Pages 26 and 27, if you can only spend 15 seconds to look for the ACmV and AC+DCmV ranges specs.

...
A documented limitation is still a limitation. 

You can call it a documented limitation if you want, but - as you yourself noted - the BM789 ACmV range is DC coupled and even though I have an extremely limited understanding of DMM circuitry, even I can understand that this range was *deliberately* not designed to measure ripple and noise from a PSU. Of course I am assuming that Brymen engineers know what they are doing, but that seems like a reasonable assumption.

Please RTFFP I could say:
"I know that I should use the higher ranges for signals like this, but when someone measures a lower level signal, and it raises up above the limit, there is no way of knowing it, you just stop seeing anything above this level, while the voltage might be skyhigh. I can not test it with higher voltages unfortunately, but would be nice if someone could do that.
So I am not vorrying about accuracy above the specified limit, but about the fact that it does not show overrange on the AC scale."

It is NOT documented however, that the meter can not show that it overranged on the AC scale under certain circumstances. (We still don't know whether this is also the case with simmetrical signals or not.)

[Caliaxy]

If I understand correctly, the chief complain here is not that the meter cannot measure outside its published specs (DC offset > 1V in AC mV mode), but that it displays erroneous values when this happens (as opposed to "OL"), tricking the unsuspecting user (other people, of course, not us, because now we know it).
...

And again, perhaps the unsuspecting user should read the fine manual, which I did even though I don't own a BM789, and it took all of 45 seconds to find the specs.
Here: http://www.brymen.com/images/ProductsList/BM780_List/BM789-5-manual-print1-r7.pdf

Thank you for the link. bdundham7 also posted a snapshot of the relevant page of the manual earlier in the thread.

Pages 26 and 27, if you can only spend 15 seconds to look for the ACmV and AC+DCmV ranges specs.

That line in the manual ("Signal peak absolute values, including DC bias, less than 1000mVpeak") does not infer that the meter would display seemingly valid but false values when overloaded. In every single other mode (DC mV included), the meter displays “.OL” when overloaded, hence the reasonable expectation to do so in AC mV as well. Nothing more, nothing less.

Again, I don’t think it’s a big deal if you are aware of it. Many other meters on the market do the same thing.

[AndrewBCN]

...
In every single other mode (DC mV included), the meter displays “.OL” when overloaded, hence the reasonable expectation to do so in AC mV as well.
...

Unfortunately the "reasonable expectation" argument doesn't work here, for a variety of reasons, but I suggest you read the excellent post by 2N3055, here:

https://www.eevblog.com/forum/testgear/brymen-bm789/msg3686746/#msg3686746

Again, you and the OP are assuming that this was an oversight by the Brymen engineering team, or a bug in the firmware of the BM789. I don't think that's the case at all.

And as has been stated many times: if one so much as spends 45 seconds to read the User Manual and use the DMM accordingly, this issue will never come up.

[bdunham7]

Unfortunately the "reasonable expectation" argument doesn't work here, for a variety of reasons, but I suggest you read the excellent post by 2N3055, here:
Again, you and the OP are assuming that this was an oversight by the Brymen engineering team, or a bug in the firmware of the BM789. I don't think that's the case at all.

We're all assuming stuff, 2N3055 included.  The 1V_pk limitation is documented.  The apparent lack of an overload indication under specific circumstances is not, so why it is that way is unknown.  As I said that could be a design issue that Brymen knows about, one they don't know about or a single-unit defect of the OPs meter. 

And as has been stated many times: if one so much as spends 45 seconds to read the User Manual and use the DMM accordingly, this issue will never come up.

The whole point of using a meter a good portion of the time is to measure an unknown (to a degree) signal.  If you are expecting ~500mV but the meter reads 650mV and the actual voltage is 30V--or 300V--that is at least a glitch. It might also just violate a standard somewhere, but I'm not sure about that.

[Fungus]

even I can understand that this range was *deliberately* not designed to measure ripple and noise from a PSU.

I wouldn't use that word for it. I doubr it was a design goal.

I'd say "knowingly". The Brymen engineers did what they could with the chipset and price point they had available.

Anything they couldn't cram in there? They documented it. What else could they do?

It might be possible to make it say 0L, we'll see. I'd bet heavily against them being able to make it measure correctly though.

[2N3055]

Unfortunately the "reasonable expectation" argument doesn't work here, for a variety of reasons, but I suggest you read the excellent post by 2N3055, here:
Again, you and the OP are assuming that this was an oversight by the Brymen engineering team, or a bug in the firmware of the BM789. I don't think that's the case at all.

We're all assuming stuff, 2N3055 included.  The 1V_pk limitation is documented.  The apparent lack of an overload indication under specific circumstances is not, so why it is that way is unknown.  As I said that could be a design issue that Brymen knows about, one they don't know about or a single-unit defect of the OPs meter. 

And as has been stated many times: if one so much as spends 45 seconds to read the User Manual and use the DMM accordingly, this issue will never come up.

The whole point of using a meter a good portion of the time is to measure an unknown (to a degree) signal.  If you are expecting ~500mV but the meter reads 650mV and the actual voltage is 30V--or 300V--that is at least a glitch. It might also just violate a standard somewhere, but I'm not sure about that.

Fact that you cannot pour hydrochloric acid in input sockets is not documented.
Fact that you cannot hit it with a AVe hammer is not documented.
Fact that you cannot plant daisies with it is not documented.
Bad, bad specs...tsk, tsk...

 
What is this, McDonalds ?  "You cannot pour hot coffee on babies" kind of thing?

As I said, in a perfect world, it would be nice that if you connect 1000V to mV input, it should not damage it, and then meter should, in a pleasant voice say: "bip. Dear operator..  please make a note that you connected meter to 1000V on a millivolt range. While I was designed to withstand such overload indefinitely, and your safety is not jeopardized, I am unable to measure voltage correctly. Please change input range to volts. Thank you for your cooperation....bip".

Are you kidding me?

Meter allread has foolproof range designed to completely autorange form 0 to 1000V. It is autoranging volt range. That is the one you use when you don't know what are you measuring. mV range is manual specialized mode for precise measurement that you use only when you know it will be less than 600mV for sure...

That is how you use that meter. Period. Everything else is either lack of knowledge (which is fine, you learn and move on) or deliberate abuse of instrument. Which is also fine if you paid it with your own money. You might as well burn it, see if I care..
But don't expect meter to do anything you want. It is operating outside it's operating envelope. All bets are off.

[bdunham7]

As I said, in a perfect world, it would be nice that if you connect 1000V to mV input, it should not damage it, and then meter should, in a pleasant voice say: "bip. Dear operator..  please make a note that you connected meter to 1000V on a millivolt range. While I was designed to withstand such overload indefinitely, and your safety is not jeopardized, I am unable to measure voltage correctly. Please change input range to volts. Thank you for your cooperation....bip".

Are you kidding me?

Every single one of my DMMs does exactly that, more or less, AFAIK--including some very, very old models.  The newer ones with CAT ratings can take that 1000V on any range--ohms, diode check, etc.  I believe it is a requirement.  As far as there being a requirement for an overload indicator, I'll defer to others since I don't have any written standards to consult.  Input protection circuitry is not that hard to make and in any case, is a well known quantity.

[Caliaxy]

...
In every single other mode (DC mV included), the meter displays “.OL” when overloaded, hence the reasonable expectation to do so in AC mV as well.
...

Unfortunately the "reasonable expectation" argument doesn't work here, for a variety of reasons, but I suggest you read the excellent post by 2N3055, here:

https://www.eevblog.com/forum/testgear/brymen-bm789/msg3686746/#msg3686746

Again, you and the OP are assuming that this was an oversight by the Brymen engineering team, or a bug in the firmware of the BM789. I don't think that's the case at all.

And as has been stated many times: if one so much as spends 45 seconds to read the User Manual and use the DMM accordingly, this issue will never come up.

I did read that post (and all the other posts in this thread). I find it excellent and educative (like most 2N3055's posts) but a little bit exaggerated (the part with 10000V on mV scale...). I didn't say it's a bug or an oversight, I said it is a design fail, which does not infer that Brymen are (or are not) aware of it.

Maybe "design fail" is too harsh. What would this be then? A price-motivated decision? It would be great if a good meter from a reputable brand would not display false information when overloaded. Grateful to learn that it might. It might not be easy to do it right. If this is understandable in lower cost meters, in Brymen's case, even their flagship model (BM869S) behaves the same way. But their flagship meter is also positioned at a very competitive price point... In A brands, the cheaper meters seem to behave similarly but at least the higher end ones behave correctly. Maybe this is the difference between A brands and B brands, and these are the kind of trade offs one should be aware of.

Again, no drama here, we can all live with this. I don't expect things to be perfect. The fact that you need to press a button 6 times to switch from AC mV to DC mV bothers me more than the DC offset issue raised here, but I can live with that too if I understand that it saves on the cost (and, of course, I'm free to choose to pay the difference if I want better).

Out of curiosity: what's your meter? Does it behave the same way? If yes, were you aware of it before this thread?

[Neutrion]

Anything they couldn't cram in there? They documented it. What else could they do?

It might be possible to make it say 0L, we'll see. I'd bet heavily against them being able to make it measure correctly though.

I don't want it to read correctly above the specified limit, only to show "OL" if overloaded.

But it is kind of amazing to see people who never ever measure anyting else than what they expect, this would be my goal as well to get to this level.
But for those who are not perfect yet, an example again:
User starts measuring correctly on the V AC scale,  to see what is the level of the signal like.
Meter shows 200 mV AC, so user proceeds to the mV scale because he needs  better accuracy.
(Up until this point I think we all agree that he is doing right...)
Than he sees the signal raising to 600mV. And there he can not be sure whether it is 600mV or maybe 325V because something went wrong.
I will do some tests to see what is the highest reading which one can trust. But it depends both on frequeny and duty cycle. Possibly also on waveform, but I can not easily test anything else than a squarewave.

If we could find out whether there is any limit up to which the meter could be trusted, than it means that it could be set as overrange limit in the FW as well. But than we would lose counts.
But I hope to get a more elegant solution.

[bdunham7]

But I hope to get a more elegant solution.

People have mentioned an external blocking capacitor, but to me that just seems like an opportunity to short something out or get zapped.  After all, the reason I have CAT-rated meters, shielded test leaks, insulated alligator clips and so on is to avoid the accidental zapping of either hapless components or myself.  So here's an idea!

Some of Fluke's older but still produced meters lack the Lo-Z function.  This feature, also known as Ghost Voltage or Stray Voltage eliminator, works by lowering the input impedance of the meter with a parallel PTC so as to short out very high impedance voltage leakage that you don't want to measure.  So, Fluke has an add-on unit, with nicely shielded jacks and plugs, that can be inserted between the test leads and the meter to add this functionality.  Note the price! 

https://www.fluke.com/en-us/product/accessories/adapters/fluke-sv225

Brymen (or someone) can introduce the same thing only put a 1kV blocking capacitor in the unit instead of a PTC.  They can call it the 'ripple master' or TrueAC or DC Block...or whatever.

[2N3055]

As I said, in a perfect world, it would be nice that if you connect 1000V to mV input, it should not damage it, and then meter should, in a pleasant voice say: "bip. Dear operator..  please make a note that you connected meter to 1000V on a millivolt range. While I was designed to withstand such overload indefinitely, and your safety is not jeopardized, I am unable to measure voltage correctly. Please change input range to volts. Thank you for your cooperation....bip".

Are you kidding me?

Every single one of my DMMs does exactly that, more or less, AFAIK--including some very, very old models.  The newer ones with CAT ratings can take that 1000V on any range--ohms, diode check, etc.  I believe it is a requirement.  As far as there being a requirement for an overload indicator, I'll defer to others since I don't have any written standards to consult.  Input protection circuitry is not that hard to make and in any case, is a well known quantity.

Ok, so your point is that it should survive.
Ok, valid point.
And it will, because it was tested it will. Survival is not in question here.

But, if the point is that it should survive,  and also show voltage is too high for the range and that there might be dangerous voltages and that you should change range?
On every range

So here is a brain teaser :  in your own words it will survive in any range including Ohms, right.  And what will meter set on ohms show when you connect it to say 400V?
It will keep showing Open on the screen. Whoops....

That is exactly why meters for this "I have no clue what to expect" thing do exist. On my BM525 you have auto mode. It will automatically detect AC, DC or Ohms/continuity.
In a single switch position.

You want failsafe?
There are are also Duspol type voltage tester devices. I have BM75EU from that type. Guess what I use when poking around electric installations, power sockets, fuses and anything 240V AC ? It also has vibration warning, RCD tester, and works even when batteries die. LCD with precise measurements and resistance/continuity stops working, but voltage detection still works. That is regulatory requirement for that type of devices.. User safety first. Best accuracy 1.5% DC, 2.5% AC.

BM789 is a electronics meter, very, very nice one. High resolution and accuracy, lots of functions. It even has "fluke like" diode test, continuity with screen blinking, it will remember settings..
It is probably best meter in the class. Even if you don't look at the price.
But it is not failsafe device for field use, it is not explosion proof, it is not IP87...
It requires operator to operate it with intelligence, otherwise garbage in , garbage out.

Don't get me wrong. I do understand that completely failsafe device probably could be made.
You would end up with a device that would be a fancy version of that Duspol type device.
No current measurement. They are inherently unsafe. Slow auto ranging/switching (because now it has to not only range but also decide which one is it supposed to do)...No high bandwidth, for better RF resilience. etc etc..

And no AC mV range.. That is how Fluke solved that problem with F87V.
I prefer to have it, even if it requires thinking when using..

When you guys find perfect meter let me know. I am interested, but in meantime I'll work with what I have.
In a  way the instrument designer expects me to do it. That guarantees good results.

Whoever doesn't like that, I have a snarky Open Source zealots like comment : " If you don't like, it make your own!".

[joeqsmith]

3055, funny post...   I want that James Bond meter.    Keep in mind that the OP just expects it to display over range, not draw the bath water for them.     
 

As I said, in a perfect world, it would be nice that if you connect 1000V to mV input, it should not damage it, and then meter should, in a pleasant voice say: "bip. Dear operator..  please make a note that you connected meter to 1000V on a millivolt range. While I was designed to withstand such overload indefinitely, and your safety is not jeopardized, I am unable to measure voltage correctly. Please change input range to volts. Thank you for your cooperation....bip".

Are you kidding me?

Every single one of my DMMs does exactly that, more or less, AFAIK--including some very, very old models. The newer ones with CAT ratings can take that 1000V on any range--ohms, diode check, etc.  I believe it is a requirement. As far as there being a requirement for an overload indicator, I'll defer to others since I don't have any written standards to consult.  Input protection circuitry is not that hard to make and in any case, is a well known quantity.

Ok, so no luck on your end.  All your meters behave as expected while sweeping the frequency, amplitude and offset.   Not at all what I would expect but I can try some of my meters and see how they behave.     

Just a friendly reminder that you may remember one of the members here damaging a 121GW PCB.  Dave suggesting that the lower voltages could not damage the meter the way that was shown.    So I setup an experiment using a DC voltage with enough current that the with the low voltage clamps engaged, it would not load down the supply.  100s of mA lethal stuff.     Basically the idea was if the switch was rotated with the input live, we could draw an arc and have enough current to burn the board. 





Be careful. 

[floobydust]

16.1 REASONABLY FORESEEABLE MISUSE
"No HAZARDS shall arise if adjustments, knobs, or other software-based or hardware-based controls are set in a way not intended, and not described in the instructions. Other possible cases of REASONABLY FORESEEABLE MISUSE that are not addressed by specific requirements in this standard shall be addressed by RISK assessment (see Clause 17). "

But the certifiers can screw up too and just re-test the function that was being tested - not all the others, so side effect failures can sneak through.
If a DMM doesn't display a hazard when it's expected, i.e. on ACV and not functioning- that's unsafe. After a transient test for example, it's expected to still display HV if present and you are intending to measure that. The owner's manual can give coverage for many misuse scenarios.


The Brymen IC is familiar to us, study the PCB a bit. For these chips, true-RMS is calculated by a tiny DSP section, which removes the DC component from the result.
So A/D reading that module's output will never know if there is a DC overload unless you switch the mux and look pre-RMS for that A/D clipping.
I can't see any rectifier though, the op-amp if used can also saturate and you have no way of knowing like in the 121GW. The PCB shows provisions for an entire IC/subsection maybe for True-RMS (four tantalums).
A firmware challenge is correctly looking at the quantity to auto-range (can't on a fixed 100mV range...) so it can display OL, can get missed.

[bdunham7]

So here is a brain teaser :  in your own words it will survive in any range including Ohms, right.  And what will meter set on ohms show when you connect it to say 400V?
It will keep showing Open on the screen. Whoops....

Actually in that case they say 'OVERLOAD' or OL.  Which is exactly what they are seeing--voltage above their range limit.  I don't expect them to do more than tell me that they can't get a reading, what I don't want to see is a normal-looking but incorrect reading.

That is exactly why meters for this "I have no clue what to expect" thing do exist. On my BM525 you have auto mode. It will automatically detect AC, DC or Ohms/continuity.
In a single switch position.

I'm not after that.  I'm only asking that the meter do one of two things--give me a reasonably correct answer or say "I don't know".  The only fail is when it gives me a plausible but completely wrong answer.  I'm sure that you could probably make any meter fail this at some point, so it just comes down to what one thinks is reasonable and what other similar meters are capable of.

And no AC mV range.. That is how Fluke solved that problem with F87V.
I prefer to have it, even if it requires thinking when using..

That's not true, the F87V has a 600 mVAC range, it is just doesn't have a separate switch position for it. 

When you guys find perfect meter let me know. I am interested, but in meantime I'll work with what I have.

OK, it's the Fluke 189.  Which is why they go on eBay for three times the price of a new BM789!   

[2N3055]

Anything they couldn't cram in there? They documented it. What else could they do?

It might be possible to make it say 0L, we'll see. I'd bet heavily against them being able to make it measure correctly though.

I don't want it to read correctly above the specified limit, only to show "OL" if overloaded.

But it is kind of amazing to see people who never ever measure anyting else than what they expect, this would be my goal as well to get to this level.
But for those who are not perfect yet, an example again:
User starts measuring correctly on the V AC scale,  to see what is the level of the signal like.
Meter shows 200 mV AC, so user proceeds to the mV scale because he needs  better accuracy.
(Up until this point I think we all agree that he is doing right...)
Than he sees the signal raising to 600mV. And there he can not be sure whether it is 600mV or maybe 325V because something went wrong.
I will do some tests to see what is the highest reading which one can trust. But it depends both on frequeny and duty cycle. Possibly also on waveform, but I can not easily test anything else than a squarewave.

If we could find out whether there is any limit up to which the meter could be trusted, than it means that it could be set as overrange limit in the FW as well. But than we would lose counts.
But I hope to get a more elegant solution.

Do you work on thermionic valve amplifiers?
That is, for instance, place where you can have 100mV AC combined with 300V. On audio amplifiers you can have 10mV riding on 80V...
But most of those high DC voltages are static working point related, i.e. don't change much or at all.
Most of them might move a bit with thermal bias circuits doing their stuff.

I cannot find scenario where it suddenly goes from 100mVAC+0 DC to 100mVAC+50V DC.
If that happens during testing, that means you switched to mV too soon, before you really figured out what voltages you have here.. Back to the square one..

Honestly, I understand you would like it to be smarter than it is. I have nothing against it. It's just I really rarely see that kind of scenario. Because of measurement practice that was beaten into me many, many years ago with moving coil, manual range meters. One wrong move, bum, you don't have a meter anymore.. I know todays meters survive pretty much anything, and that you can switch between functions without disconnecting them etc.. But I still don't do it. And I am rewarded with a fact that i don't see those fringe cases where meters behave weirdly.
I'm not saying you're wrong, but pointing out that you CANNOT trust meter blindly EVER. Measure 3 times, cut once.

While we are talking real world scenarios, when, for instance, repairing something, my first troubleshooting start with meter in volts (autorange) and checking power busses. If they look kinda OK,  I fire up scope. And check those power busses again with a scope to see if they are clean.
Meters are utterly useless to see actual SIGNALS. They come in handy when I first see, with a scope, there is a clean DC voltage on a point and IF I want to see precisely what it is.
To add insult to the injury, most of the time scope measurements will do good enough job, and it can measure dozens of parameters simultaneously.
Meter is more used for continuity, resistance, and when more accuracy is needed. Which also is a moot point when you have 16bit 0.25% accurate scope..
My method is to poke around circuit to understand it. That way there aren't much surprises.

Every repair job is actually reverse engineering. That or you simply stab in a dark and change random components, or just change them in a bulk.

When you're designing your own circuits, there should not be such unexpected surprises. Otherwise you're doing something wrong.
If there is a scenario where I suspect that (because of the circuit) there can be malfunction that can inject some weird stuff, i connect two meters, one measuring main parameter, other one monitoring for error conditions. Or watch it with a scope. Truth is you should always go back and forth and repeat measurement, varying methods, until you're sure you're measuring the right thing.

Best,

[bdunham7]

Ok, so no luck on your end.  All your meters behave as expected while sweeping the frequency, amplitude and offset.   Not at all what I would expect but I can try some of my meters and see how they behave. 

No, I'm just referring to their ability to withstand without exploding.  Getting any of them to read 'wrong' is easy with a combination of high crest factor and offset.  I even had my 8846A reading what should be a DC signal as the wrong polarity, until I turned on the analog filter and it started behaving.  Finding a signal that baffles something like the F189 but not the the Harbor Freight one is the trick.  I'm down to my last HF meter anyway....

Edit:  We would have to agree on what the second meter should be so that any results can be replicated.  So far, the 8846A has easily been beaten "Harbor Freight freebie outperforms Fluke's best bench meter!" because it starts by default with the analog filter off.  A 0-1V 5% 1kHz test signal, which should read ~50mV, is 15.4mV on the 8846A, 46.4mV on the HF.  The 46.4mV is more or less correct because if you turn the analog filter on or use a 189/289, that's what you get.  I was going to suggest the 189 as the target because you have one and they're well regarded, but I'm finding it very difficult to fool. 

[2N3055]

So here is a brain teaser :  in your own words it will survive in any range including Ohms, right.  And what will meter set on ohms show when you connect it to say 400V?
It will keep showing Open on the screen. Whoops....

Actually in that case they say 'OVERLOAD' or OL.  Which is exactly what they are seeing--voltage above their range limit.  I don't expect them to do more than tell me that they can't get a reading, what I don't want to see is a normal-looking but incorrect reading.

That is exactly why meters for this "I have no clue what to expect" thing do exist. On my BM525 you have auto mode. It will automatically detect AC, DC or Ohms/continuity.
In a single switch position.

I'm not after that.  I'm only asking that the meter do one of two things--give me a reasonably correct answer or say "I don't know".  The only fail is when it gives me a plausible but completely wrong answer.  I'm sure that you could probably make any meter fail this at some point, so it just comes down to what one thinks is reasonable and what other similar meters are capable of.

And no AC mV range.. That is how Fluke solved that problem with F87V.
I prefer to have it, even if it requires thinking when using..

That's not true, the F87V has a 600 mVAC range, it is just doesn't have a separate switch position for it. 

When you guys find perfect meter let me know. I am interested, but in meantime I'll work with what I have.

OK, it's the Fluke 189.  Which is why they go on eBay for three times the price of a new BM789!   

It will show whatever it shows when resistance is too high. Like with infinite resistance. Or in a case of BM869, it will start blinking trying to autorange.
Not at any moment it will give indication that you connected VOLTAGE to the inputs. That is my point.

Without that kind of Auto select mode it cannot know that what you connected to it is wrong. Except in current mode where you have fuse as an indicator.

As for F87V I apologize, I meant mV DC. My bad. And you know, despite that what I meant, but never mind. I said at the beginning, this happens only if you mix AC and DC.
If you feed it only AC (AC coupled like F87V) there is no problem and will nicely show OVL.

I don't trust used old stuff that much. Not even Fluke, especially when I saw with my own eyes what people do to them.

In my book both BM869 and MTX3293 come pretty close to fantastic... Each in it's own right.

By the way MTX 3293 cannot have that problem because it is single input, single function, and is AC coupled from uV to kV. In AC+DC is DC coupled but uses both to autorange.

[2N3055]

I did read that post (and all the other posts in this thread). I find it excellent and educative (like most 2N3055's posts) but a little bit exaggerated (the part with 10000V on mV scale...). I didn't say it's a bug or an oversight, I said it is a design fail, which does not infer that Brymen are (or are not) aware of it.

Thank you for the nice words and sorry for the drama.. I tend overexaggerate with comparisons to prove the point works at any scale, my bad.

[bdunham7]

It will show whatever it shows when resistance is too high. Like with infinite resistance. Or in a case of BM869, it will start blinking trying to autorange.
Not at any moment it will give indication that you connected VOLTAGE to the inputs. That is my point.

OK, true--but that would be an expectation and feature that go beyond what is required and what both the OP and I would expect, which is that the meter simply not display a plausible but grossly wrong number.  It's not just a safety issue, although we like to talk about those dramatically.  Suppose I'm measuring a single-ended square wave like the OP at 300mVAC and it jumps up to 900mVAC?  I expect to see 'OL', not 640mVAC.  I don't think that's unreasonable.

As for not trusting old stuff, I'm in the opposite camp, although I may agree if it is old abused junk.  To me 'new' means 'experimental'.  I start to trust things after a decade or so of flawless service.  And I've been proven right more than once...

[Caliaxy]

And no AC mV range.. That is how Fluke solved that problem with F87V.
I prefer to have it, even if it requires thinking when using..

You got it wrong. Like BM789, Fluke 87V does have an AC 600mV range (and a DC 600mV range), but they are not on the same rotary dial position. The AC 600mV is on the AC V position, which starts auto-ranging from 600mV up, whereas the DC 600mV range has its own rotary dial position, all for itself. The DC V mode starts auto-ranging from 6V up. All AC V ranges (including 600mV) are AC coupled, so the DC offset is irrelevant, unlike in BM789.

What 87V doesn't have is a combined AC+DC mV mode, hardly useful for many but plagued by the DC offset issue (which I'm starting to believe is a ubiquitous problem across most brands).

[AndrewBCN]

...
I didn't say it's a bug or an oversight, I said it is a design fail, which does not infer that Brymen are (or are not) aware of it.

Maybe "design fail" is too harsh. What would this be then?
...

Seems to me it's quite simply this:
1. Don't read the BM789 User Manual (or pretend that you didn't read it).
2. Select the ACmV 600mV range even though you know full well you are going to exceed the maximum measurable voltage for that range
3. Carefully select a large DC offset that will saturate the DC coupled ACmV input circuitry of the DMM, thus preventing the detection of an overload condition.
4. Bitch about it on the EEVblog. 

[joeqsmith]

Ok, so no luck on your end.  All your meters behave as expected while sweeping the frequency, amplitude and offset.   Not at all what I would expect but I can try some of my meters and see how they behave. 

No, I'm just referring to their ability to withstand without exploding.  Getting any of them to read 'wrong' is easy with a combination of high crest factor and offset.  I even had my 8846A reading what should be a DC signal as the wrong polarity, until I turned on the analog filter and it started behaving.  Finding a signal that baffles something like the F189 but not the the Harbor Freight one is the trick.  I'm down to my last HF meter anyway....

Edit:  We would have to agree on what the second meter should be so that any results can be replicated.  So far, the 8846A has easily been beaten "Harbor Freight freebie outperforms Fluke's best bench meter!" because it starts by default with the analog filter off.  A 0-1V 5% 1kHz test signal, which should read ~50mV, is 15.4mV on the 8846A, 46.4mV on the HF.  The 46.4mV is more or less correct because if you turn the analog filter on or use a 189/289, that's what you get.  I was going to suggest the 189 as the target because you have one and they're well regarded, but I'm finding it very difficult to fool. 

I tried a few.  The Gossen I have is 300,000 count and there's no mV range.   So I can't count that one. 

121GW,  UNI-T UT61E+, Fluke 107, CEM DT9939 all have problems which I would expect.  I'm just using a sine.   Again, for me, no big deal. 

For a list of the meters I still have, you can use the spreadsheet and see beyond the free HF and 189, if we have any other common ground.   That or post a list of what you have. 

But I hope to get a more elegant solution.

People have mentioned an external blocking capacitor, but to me that just seems like an opportunity to short something out or get zapped.  After all, the reason I have CAT-rated meters, shielded test leaks, insulated alligator clips and so on is to avoid the accidental zapping of either hapless components or myself.  So here's an idea!
....

If working in CAT III, I am dealing with AC mains.  The meter I use has no mV and clamp inputs.  It can't read resistance, capacitance, continuity...  I don't go stringing caps around and there would be no reason for it.  If the mains start having a DC bias, all hells breaking loose.    Still, it would be trivial to mount a cap in an insulated housing with proper connectors.  I've never felt the need to do it.   

***
Added pictures.   

[bdunham7]

That or post a list of what you have. 

Other than stuff that can be dismissed as too old (which is most of my stuff including me...) I have a Fluke 116, 189, 289, 323 and 8846A.  I also have an HP 34401A, 3478A and several HF-level meters including the free model.  That is all unless I've forgotten something.

If the mains start having a DC bias, all hells breaking loose.    Still, it would be trivial to mount a cap in an insulated housing with proper connectors.  I've never felt the need to do it.

True, but I would think of something mains-derived, like a power supply, where a rectifier failure could change the picture quite quickly.  I don't need to do that either because I have meters with their own blocking caps.

[joeqsmith]

So can I can get the BM789 to read 0 mVAC with a signal applied as I suggested.  You bet.  Do I care, other than for a small bit of entertainment, not at all.        I could run others but will wait and see what bdunham7 comes up with.   

[AndrewBCN]

And no AC mV range.. That is how Fluke solved that problem with F87V.
I prefer to have it, even if it requires thinking when using..

You got it wrong. Like BM789, Fluke 87V does have an AC 600mV range (and a DC 600mV range), but they are not on the same rotary dial position. The AC 600mV is on the AC V position, which starts auto-ranging from 600mV up, whereas the DC 600mV range has its own rotary dial position, all for itself. The DC V mode starts auto-ranging from 6V up. All AC V ranges (including 600mV) are AC coupled, so the DC offset is irrelevant, unlike in BM789.
...

No, 2N3055 got it right, the AC V position on the Fluke 87V has 6000 count resolution, whereas the separate BM789 ACmV position has 60,000 count resolution. If you use the Auto position on the BM789 it will autorange exactly the same as the Fluke 87V.

That means the Brymen has a high resolution, separate dial position for manual range 600mV ACmv which you can choose to use for high resolution ACV measurements - exactly as 2N3055 wrote. And yes, they should add in the BM789 User Manual a warning that this range requires thinking... 

[AndrewBCN]

I can see the headlines tomorrow:
Electronics Engineer Exposes Major Design Flaw In Hundreds of Multimeters
Proves That Most DMMs Cannot Be Used To Measure Ripple And Noise In PSUs

[bdunham7]

I could run others but will wait and see what bdunham7 comes up with.

Don't look at me!  All I have so far is a Fluke advertisement.  I was unable to get any of my meters other than the HF freebie to make an error with this signal (I used 1kHz)  They all read correctly up to a point then displayed an overrange indication.  Even the CenTech P37772 more or less got it right, however that doesn't have a mVAC range, but it does have 2 VAC.  The HF freebie made a similar error at a  higher AC voltage, which was needed since its lowest range is 200VAC.  You know, since it is intended primarily a mains and service panel meter.

So the goal is to fool a 'good' meter with a signal that the HF meter can read correctly, whether by proper operation or luck.  We really need to use the DC ranges, because of the lowest AC range being 200V.  I've tried low duty cycle DC pulses, which defeated the 8846A w/o analog filter on, but I have been completely unable to fool the 189, other than going beyond bandwidth (I had a 50Hz 0.01% duty cycle pulse, for example).  So if anyone can come up with a signal that the 189 gives a 'wrong' result on in the DC range....

[floobydust]

I can see the headlines tomorrow:
Electronics Engineer Exposes Major Design Flaw In Hundreds of Multimeters
Proves That Most DMMs Cannot Be Used To Measure Ripple And Noise In PSUs

Na. We have the technology! Like a built-in IQ test

[bdunham7]

No, 2N3055 got it right, the AC V position on the Fluke 87V has 6000 count resolution, whereas the separate BM789 ACmV position has 60,000 count resolution. If you use the Auto position on the BM789 it will autorange exactly the same as the Fluke 87V.

That's preposterous.  The bottom 10% of a 60,000 count range is not equivalent to a full 6000 count range, especially when you are talking about TRMS.  And the 87V has a 'hi-res' mode, but I don't have one so I can't tell you the details of that.  But it's OK to be silly if you like--this is EEVBlog after all.

[bdunham7]

So can I can get the BM789 to read 0 mVAC with a signal applied as I suggested. 

So it appears to completely clamped yet using only the calculated AC component to determine whether to display OL, if I'm understanding things correctly.  Or did you have to dial that in more carefully?  Could you try and replicate the OPs issue?  I'd say a 0-1V 50% 200Hz signal and then vary the amplitude up and down and see if you can get it to 'hang' at one value, and if it changes when you swap the leads.

[Caliaxy]

So can I can get the BM789 to read 0 mVAC with a signal applied as I suggested.  You bet.  Do I care, other than for a small bit of entertainment, not at all.        I could run others but will wait and see what bdunham7 comes up with.

Well, I'm not bdunham7 but, as mentioned in some of my posts above, most of the meters I tried have the same issue: Fluke 101, 107, 17B+, Uni-T UT61e, Agillent U1252A, BM235, BM689S. Fluke 189, 89IV and Agillent U1282A seem to work fine. I don't care much either, but it's good to know. I almost always use an old HP analog meter for AC mV measurements. Overloaded it a few times with voltages higher than the selected range, but its needle is still intact 

[floobydust]

AN8008 ACV bugs: Input 90mVAC on the mV AC range, no DC offset (autoranging enabled 9.999mV and 99.99mV ranges). Displays: 90.00 this is OK.
As you increase a +ve or -ve DC offset, the reading suddenly drops to say 15.00 then additional DC offset leads to display in a loop showing: 40.30, 0.000, 00.00, OL, and constantly repeating.
If you manual range to 99.99mVAC, you get a correct reading 90.00mVAC but this drops and even goes to 00.00mVAC with increasing offset. With no DC offset it returns to the correct reading.
If you now enable autoranging, the multimeter is lost and reads as if on manual range, but no longer does the autorange cycling and can read silly values.
You have to switch to another function or reboot to stop this.

Mentioning this as it is a test of the OL and autorange algorithms that commonly have bugs.

[joeqsmith]

... most of the meters I tried have the same issue: Fluke 101, 107, 17B+, Uni-T UT61e, Agillent U1252A, BM235, BM689S. Fluke 189, 89IV and Agillent U1282A seem to work fine. I don't care much either, but it's good to know. ...

I'm sure I could find a few in the my mix that would handle the ACmV fine but I would guess I could find other problems with them.   I'll typically gloss over what I consider fringe cases. 

Now if you make a point to advertise your meter has a frequency counter that can read up to 220MHz.  You're so proud of it, that you place it right on the box.  And I happen to watch Daves review and see your bright idea was to protect the input with a couple of series PTCs that any EE is going to know they will act like capacitors and you didn't limit the current through them.  Well,  I'm not going to let that go.  It's not a fringe case when you claimed it would do something.   I'm going to push it until it works, or goes up in flames.       

[bdunham7]

I can see the headlines tomorrow:
Electronics Engineer Exposes Major Design Flaw In Hundreds of Multimeters
Proves That Most DMMs Cannot Be Used To Measure Ripple And Noise In PSUs

...but trusty old Fluke 87V gets the job done once again and Fluke sells another million units!   

[bdunham7]

most of the meters I tried have the same issue: Fluke 101, 107, 17B+, Uni-T UT61e, Agillent U1252A, BM235, BM689S.

Do you mean they all displayed wrong readings or did they display an overload indicator?

[Caliaxy]

And no AC mV range.. That is how Fluke solved that problem with F87V.
I prefer to have it, even if it requires thinking when using..

You got it wrong. Like BM789, Fluke 87V does have an AC 600mV range (and a DC 600mV range), but they are not on the same rotary dial position. The AC 600mV is on the AC V position, which starts auto-ranging from 600mV up, whereas the DC 600mV range has its own rotary dial position, all for itself. The DC V mode starts auto-ranging from 6V up. All AC V ranges (including 600mV) are AC coupled, so the DC offset is irrelevant, unlike in BM789.
...

No, 2N3055 got it right, the AC V position on the Fluke 87V has 6000 count resolution, whereas the separate BM789 ACmV position has 60,000 count resolution. If you use the Auto position on the BM789 it will autorange exactly the same as the Fluke 87V.

That means the Brymen has a high resolution, separate dial position for manual range 600mV ACmv which you can choose to use for high resolution ACV measurements - exactly as 2N3055 wrote. And yes, they should add in the BM789 User Manual a warning that this range requires thinking... 

In my post #24 above (replying to 2N3055's post #21) I was joking (half seriously) that Fluke 87V probably avoided the problem under discussion here by not having a dedicated AC mV position on the dial at all (and moving it under the AC V mode, which is AC coupled). After reading his reply #79 I worried that my reply #24 misled him into believing that Fluke 87V doesn’t have an AC mV range at all (which is not true but might have been an interpretation of what I wrote in #24), so I tried to clarify. That’s the reason of my reply #88, which you (wrongly) disagree with.

#88 wasn’t intended to be malicious. I’m not sure why you tend to see everything this way. 

[Caliaxy]

most of the meters I tried have the same issue: Fluke 101, 107, 17B+, Uni-T UT61e, Agillent U1252A, BM235, BM689S.

Do you mean they all displayed wrong readings or did they display an overload indicator?

They all display meaningless numbers instead of OL, exactly like BM789. Forgot to (re)mention GW121.

[joeqsmith]

I would hate to think I left a few of you with the idea the having a meter show zero volts while in it's ACmV mode is unique to Brymen's new BM789,  here is another one for the UNI-T fan boys.   

Of course, switch it over to tri-display mode and we get a feel for what's really going on.   Lucky for me, the battery was charged this time when I went to use it.       Again, this meter has been damaged, repaired and then the PCB was modified but I doubt any of that would have an effect on these measurements. 

[AndrewBCN]

No, 2N3055 got it right, the AC V position on the Fluke 87V has 6000 count resolution, whereas the separate BM789 ACmV position has 60,000 count resolution. If you use the Auto position on the BM789 it will autorange exactly the same as the Fluke 87V.

That's preposterous.  The bottom 10% of a 60,000 count range is not equivalent to a full 6000 count range, especially when you are talking about TRMS.  And the 87V has a 'hi-res' mode, but I don't have one so I can't tell you the details of that.  But it's OK to be silly if you like--this is EEVBlog after all.

I don't have a 87V either but I can read (the online user manual): the Hi-Res mode does not work with the ACV ranges.
So, as I wrote, if you set the BM789 to Autorange ACV position, it will autorange exactly the same as the 87V. The main difference with the 87V in that respect (ACV measurements) is that the BM789 provides an extra, separate, high-resolution, dc-coupled ACmV manual range, which as 2N3055 noted, requires a little bit of thinking to be used.

As for being "silly", I am sorry to say but that's one way to describe this entire thread. So I guess a little bit of humor is not entirely out of place. 

[2N3055]

I go to bed, and children in other time zones get in a fight right away..

Look, people. This topic is chock full of emotions and many hurried responses without doing homework.
So let's sumarize:

- Meters that do AC+DC type of measurements (combined) do that by keeping front end DC coupled.
- Meters SHOULD AC couple in AC only measurements so you could measure 20mV AC riding on top of 100V
- For some reason Brymen doesn't do that on AC mV range. That is NOT good way to do it.
- Because of that, if you plan to measure 20mV AC riding on top of 100V using mV range, on Brymen you have to use external capacitor. Like Joe said, just throw it in a box, if you do it on a regular basis.
- There are other meters that do the same thing as Brymen and also those that have AC mV range properly AC coupled. Make note that there is no instrument available that can measure in single AC+DC measurement (single or dual display) that can do mixed scales for AC and DC.
- Would I like that Brymen did it differently, so it also AC couples when AC only mV? Yes, of course I would. That is pretty much only thing they didn't do perfectly. On my BM869 I miss that and fact that it doesn't remember preferences between powerups.
- Is it a problem in practice? It might be annoying but not a problem if you know it. Would it be better if they did AC coupling? Yes.
- Overload indicator gets confused by DC offset overdrive only. AC overdrive without offset works perfectly. It will show OVL.
- Whole problem is caused by DC offset overloading input stages.

All of this applies only to dedicated mV AC measurements. Volt ranges properly AC couple and work perfectly.
At all times meter is safe and won't be damaged. Measurements won't be valid if operated outside operating envelope.

That is summary.

[AndrewBCN]

Let's summarize (again):
1. The OP found a way to "fool" his shiny new Brymen BM789 when set to the DC-coupled high-resolution ACmV range. This basically involves saturating the input circuitry such that the DMM cannot measure the (small) AC component because of a (large) DC bias. Under these conditions the DMM neither detects an overload condition nor provides a correct measurement of the (small) AC component.
2. joeqsmith has demonstrated that a number of different DMMs from different brands can be "fooled" using a similar method.

I think the lesson here, if there is one, is that using a DMM  (any model from any brand) to get correct measurements always involves a minimum of thinking - as 2N3055 reminded us. As well as spending a few seconds to read and understand the User Manual.

Edit: Sorry 2N3055, we seem to have posted our summaries almost simultaneously. I defer to yours which is much better than mine.

Edit2: I can think of a number of reasons why Brymen did not include a series capacitor in the ACmV range front end, and similarly for other DMMs from other brands. But that would be a matter for (reasoned) discussion in another thread, if you ever want to. I for one would be very interested in your opinion.

[joeqsmith]

...
- Meters SHOULD AC couple in AC only measurements so you could measure 20mV AC riding on top of 100V
...
- Because of that, if you plan to measure 20mV AC riding on top of 100V using mV range, on Brymen you have to use external capacitor. Like

Typical meter will have a your 1k + PTC and clamp at over a kV.   The mV function could very well clamp at much lower.  UT61E for example, with  mV selected, the signal goes through PTC1 and it then clamped by Q1,8.

https://electronicshelponline.blogspot.com/2015/10/uni-ut61e-digital-multi-tester-schematic.html

The block should handle this and I would expect to see a 1kV part in use.  You should be able to measure your 20mV on top a 1kVDC.

But then again, I like that low frequency response that I can only get with DC coupling.....  Ah the choices we have. 

***
Looks like Fluke knows they need a 1kV part in there, even on their low end meters.   Looking at 300mVAC (wanted to show the 2nd meter reading something) riding on top of a 750VDC.    I'm sure a kV would be possible but I'm a bit concerned with my setup.   

Gotta say, I still like the 189.  Just a nice, basic meter.  Sure it's a relic and could use a few improvement but I bought one for the sole purpose to transient test.  While it had been badly abused before I got it, it took everything I threw at it.   

[2N3055]

Let's summarize (again):
1. The OP found a way to "fool" his shiny new Brymen BM789 when set to the DC-coupled high-resolution ACmV range. This basically involves saturating the input circuitry such that the DMM cannot measure the (small) AC component because of a (large) DC bias. Under these conditions the DMM neither detects an overload condition nor provides a correct measurement of the (small) AC component.
2. joeqsmith has demonstrated that a number of different DMMs from different brands can be "fooled" using a similar method.

I think the lesson here, if there is one, is that using a DMM  (any model from any brand) to get correct measurements always involves a minimum of thinking - as 2N3055 reminded us. As well as spending a few seconds to read and understand the User Manual.

Edit: Sorry 2N3055, we seem to have posted our summaries almost simultaneously. I defer to yours which is much better than mine.

Edit2: I can think of a number of reasons why Brymen did not include a series capacitor in the ACmV range front end, and similarly for other DMMs from other brands. But that would be a matter for (reasoned) discussion in another thread, if you ever want to. I for one would be very interested in your opinion.

Thank you for the kind words.
I would not mind if you started that discussion, but in my opinion it would necessitate at least partial R.E of front end and switching circuit. Otherwise we would all have many ideas, but no confirmation. I guess Joe did quite some work on that already..
It might even be that meter could be bodged with a capacitor as an afterthought. But I would be very reluctant to do that to my meter and also to publish it on Internet. I have same opinion on that as Joe has about his "hardenings" of various meters. Maybe that is a Pandora box better be left unopened.
Best,

[2N3055]

...
- Meters SHOULD AC couple in AC only measurements so you could measure 20mV AC riding on top of 100V
...
- Because of that, if you plan to measure 20mV AC riding on top of 100V using mV range, on Brymen you have to use external capacitor. Like

Typical meter will have a your 1k + PTC and clamp at over a kV.   The mV function could very well clamp at much lower.  UT61E for example, with  mV selected, the signal goes through PTC1 and it then clamped by Q1,8.

https://electronicshelponline.blogspot.com/2015/10/uni-ut61e-digital-multi-tester-schematic.html

The block should handle this and I would expect to see a 1kV part in use.  You should be able to measure your 20mV on top a 1kVDC.

But then again, I like that low frequency response that I can only get with DC coupling.....  Ah the choices we have. 

***
Looks like Fluke knows they need a 1kV part in there, even on their low end meters.   Looking at 300mVAC (wanted to show the 2nd meter reading something) riding on top of a 750VDC.    I'm sure a kV would be possible but I'm a bit concerned with my setup.   

Gotta say, I still like the 189.  Just a nice, basic meter.  Sure it's a relic and could use a few improvement but I bought one for the sole purpose to transient test.  While it had been badly abused before I got it, it took everything I threw at it.

Well said.

If they would restart making F189, that one I would buy right away.
If I was lucky to have bought one before, when available, I would still have it and keep it.
But I have no appetite for lottery of buying used.

[Fungus]

Gotta say, I still like the 189.  Just a nice, basic meter.  Sure it's a relic and could use a few improvement but I bought one for the sole purpose to transient test.  While it had been badly abused before I got it, it took everything I threw at it.

If they would restart making F189, that one I would buy right away.

187 is even better - same meter but no pesky supercap to worry about.

[bdunham7]

it will autorange exactly the same as the 87V.

If you are going to make such arguments, at least be honest about it instead of acting like a marketing or political spin doctor.  What you are implying is that since the BM789 has 10X the counts (the almighty counts!) of the 87V, you can consider the 6.0000V range to be the usable equivalent of the 600.0mV range of the latter since they appear to have the same resolution (100uV).  But read footnote 1 of the excerpt that I posted.  The bottom 10% of the range is 'unspecified'.  Maybe if you characterize your meter you might imagine you are getting some useful information out of that range, but don't tell me that it is 'exactly the same' or even 'just as good'.

[joeqsmith]

187 is even better - same meter but no pesky supercap to worry about.

I have replaced the SC on all four meters.  No big deal. 

If they would restart making F189, that one I would buy right away.
If I was lucky to have bought one before, when available, I would still have it and keep it.
But I have no appetite for lottery of buying used.

Same.  Maybe a better display and handle the part obsolescence is all.   

The one on the right was given to me by a good friend.  I'm the third owner.  The one on the left cost me about $120 non-working as is for parts.   Bought it to transient test.  It turned out to be in very good of condition and felt it would upset the Fluke collectors if I put a classic meter into an early grave.     The last one I brought home was a similar story.  Bought it as non-working as is. It's case looked mint which is why I bought it.  I think $60 which seemed fair for good used case.  But when I opened it up,  it was mint inside as well.  I have no plans to buy another no matter how good the deal seems so I'll pass my 189 finders luck onto you. 

[bdunham7]

That is summary.

Looks right, just the facts.  Which I think is what the OP was looking for.  Nobody replicated his specific test, but it seems clear that his issues are primarily because of the design.  And until this thread I didn't know that the mVAC range on my own F116 is also DC-coupled, although none of the other issues affect it.

But I have no appetite for lottery of buying used.

And some have no appetite for the lottery of buying new! 

[Caliaxy]

I don't have a 87V either but I can read (the online user manual): the Hi-Res mode does not work with the ACV ranges.

Not true.

[joeqsmith]

Vintage 1980's Fluke 8506A thermal RMS meter I saved from scrap.  Its set to high accuracy mode and shown with an external blocking capacitor.    My new 87V in high res mode.  Signal is a little closer to the 20mV noise mentioned above, with 800VDC offset.

Eventually the MOVs would clamp the input and we limit the current but I wonder if these caps would survive....  I'll stick with my old habits of using an external block.   It does look a bit scary hanging out there though...   

[AndrewBCN]

@Joe

That external blocking capacitor looks like it could hold a lethal amount of energy!

The question is, on the DMMs that use an internal blocking capacitor, how do they discharge it before you go on to the next measurement?

[joeqsmith]

@Joe

That external blocking capacitor looks like it could hold a lethal amount of energy!

The question is, on the DMMs that use an internal blocking capacitor, how do they discharge it before you go on to the next measurement?

 
Assuming worse case,  1/2CV^2,  0.005uF @ 20KV is a Joule.   This is the upper end of Class 3.2b.  The part would normally be used with Class 3.1d (< 0.25J).    There's some small risks when experimenting but I'm not too concerned about discharges from this part.   

I don't see any need to discharge the internal block before changing measurements. What would be your concerns?  Seems like having the low voltage clamps engaged and connecting to a +1kVDC source, then instantly measuring -1kV would provide the most stress to the meter.    Well, then again, rotating the switch with a 1kVDC source my light it up pretty good.  Outside of my rebuttal videos for Dave,  I've never tried it for real.  I assume people making such measurements are well educated, use the proper tools and think before they act ....   

Then again, the beginner hooks their new meter to the output of a MOT.  Surely it can handle a couple kV as I saw this guy Joe put 6kV into his.     

DOE standard
https://www.standards.doe.gov/standards-documents/1000/1092-BHdbk-2013/@@images/file

Datasheet for part in question
https://www.plasticcapacitors.com/images/com_sellacious/products/attachments/57/OF200.pdf

[AndrewBCN]

...

The question is, on the DMMs that use an internal blocking capacitor, how do they discharge it before you go on to the next measurement?

...
I don't see any need to discharge the internal block before changing measurements. What would be your concerns?
...

Joe, thank you very much for your detailed and perfectly documented (as always) answers. My concern is not really with an internal blocking capacitor damaging the high impedance (10MOhm?) DMM input circuit, but what would happen in the following measurements, if it could damage the circuit under measurement (presumably at a different DC potential), in other words, if the blocking capacitor inside the DMM would discharge back through the leads into the circuit under measurement?

[joeqsmith]

Oh, now there's something I had not considered.      Say low voltage clamp is active, you make your kVDC noise measurement like shown, then immediately attach the probe to your 3.3V CMOS input....  10nF with a 2kohm source at a kV.... Interesting...

[Kleinstein]

The voltage in the DC blocking cap can add the external voltage. Especially bad when you change sign of the DC part.  This way I once killed the input stage of a scope:  first measure ripply (rather low) at a +150 V supply with AC coupling and a x 1 probe, and than the same with a negative -150 V. This was too much for a scope that showed 200 V max.    Afterwards you are smarter, but still a bit surprised that so little extra voltage from the internal cap can damage the input.

I think the AC input behind the coupling capacitor could be more like 1 M (maybe even 10 M in a handheld) input resistance and I don't expect a clamp between the resistor and cap.

[Fungus]

10nF with a 2kohm source at a kV.... Interesting...

Doesn't seem like it could damage much ... but your grill starter has killed a few meters that have some input protection on them (high speed clamps after a protective resistor).

[joeqsmith]

Thanks for posting.

Not sure off the top of my head how not having the low voltage clamp would ever work in the mVAC mode with a robust handheld meter.   Maybe measure the current through the meter with the mVAC mode selected and look for the knee where the clamp starts to conduct.    Most of handhelds I have are DC coupled and could easily be measured with my bench supply. 

Still, I think we now have our first documented case and I wouldn't be surprised if there were not others.  Recently I had made a video where I used a cheap VNA to make some PDN measurements.   These are normally made with the unit turned off as well as powered up.   The VNA may not like getting exposed to the PDNs voltages, so wideband blocks are added.   You need to be very careful when bringing up the voltages and well as when turning them off.   No big deal for a $50 VNA.  Wouldn't want to tell my boss I damage a quarter million dollar VNA this way.   

[joeqsmith]

10nF with a 2kohm source at a kV.... Interesting...

Doesn't seem like it could damage much ... but your grill starter has killed a few meters that have some input protection on them (high speed clamps after a protective resistor).

Your talking about products that I would expect to be designed to handle some basic transients.  Now I'll certainly be the first to say that not all handheld meters have the same level of robustness.  And yes, there are some down right crap ones that the stupid grill starter with it's what, 5A peak waveform has damaged.     

Here we are talking about the meter becoming the source of the transient and your electronics may not be able to handle the transient it could produce.  I wouldn't be at all surprised if its happened a fair amount and people may not have have understood the cause.    Hard to say.   

Could the free meter from HF be damaged with a Fluke 189, charged to 1.5kV??   
***
Talk about your fringe case! 

[Fungus]

Could the free meter from HF be damaged with a Fluke 189, charged to 1.5kV??   
***
Talk about your fringe case! 

It would be interesting to see if any manufacturers have thought about this in their designs. Is this a place where the mighty Fluke can differentiate themselves by having an internal bleeder resister where other manufacturers don't? Will it be the other way around...?

If you've got the volts and probes at hand then it's a simple job to charge up a meter then discharge it and look at the waveform.

[bdunham7]

I think you'd have to start looking at some schematics to see if this is even a reasonable possibility.  For example, with the (original) F87, if you leave the meter in the AC range you might get, at the very most, a 0.01uF capacitor discharging through about a 10M resistor in parallel with a ~1pF capacitor.  If you switch to any other range, the capacitor is discharged instantly.  I think bench meters with a lower frequency limit, larger blocking caps and a 1M AC input impedance would be more likely to deliver such a jolt, if that is even possible.

[joeqsmith]

Could the free meter from HF be damaged with a Fluke 189, charged to 1.5kV??   
***
Talk about your fringe case! 

It would be interesting to see if any manufacturers have thought about this in their designs. Is this a place where the mighty Fluke can differentiate themselves by having an internal bleeder resister where other manufacturers don't? Will it be the other way around...?

If you've got the volts and probes at hand then it's a simple job to charge up a meter then discharge it and look at the waveform.

Bleed and high impedance don't mix.  Maybe change your habits to discharge the meter.  All these years and still new tricks to learn.      

Using a sting of LEDs to look at the waveform as well as conducting the finger test...

[AndrewBCN]

Thank you for yet another great video, Joe.

My takeaway from this thread, apart from the information that some DMMs use a blocking capacitor in their ACmV input circuit and others don't (in other words, some are AC-coupled and others are DC-coupled in their ACmV range): remember to always switch on my brains before I switch on the DMM to make a measurement. 

[bdunham7]

Bleed and high impedance don't mix.  Maybe change your habits to discharge the meter.  All these years and still new tricks to learn.      

Using a sting of LEDs to look at the waveform as well as conducting the finger test..

Interestingly I was easily able to replicate this and found that even if you switch to DC, the capacitor retains its charge.  If, for example, you go to measure 500V but the meter is in mVAC, if you switch to DC and measure your 500VDC, if you then remove the test leads and switch back to mVAC, you get the jolt.

[floobydust]

If the cap is on the high-side of the divider chain, it charged up to the DC component and the next action you take can cause damage. Moving the probe to a different circuit node, the cap discharges into it and any semi is usually killed due to the voltage.
Moving the probe to GND or a low resistance point to GND discharges the cap into the test equipment front-end. There are many RMS-converters like AD636 etc. in the graveyard as this kills their input, even though the blocking cap is post-divider string. There is no protection at that node.

With a scope, switching from AC to DC coupling shorts the blocking cap and the scope's switch makes a loud ka bang and arc. Tektronix includes a small series resistor.
The cap seems to be 27nF-220nF depending on the low freq. response of the instrument.
My practice is to touch the probe to a 1MEG resistor to GND and wait for the cap to discharge before proceeding.

[Fungus]

Cool!

I bet even Dave didn't know this. 

Using a sting of LEDs to look at the waveform

Is that the correct collective noun for LEDs?

[joeqsmith]

Bleed and high impedance don't mix.  Maybe change your habits to discharge the meter.  All these years and still new tricks to learn.      

Using a sting of LEDs to look at the waveform as well as conducting the finger test..

Interestingly I was easily able to replicate this and found that even if you switch to DC, the capacitor retains its charge.  If, for example, you go to measure 500V but the meter is in mVAC, if you switch to DC and measure your 500VDC, if you then remove the test leads and switch back to mVAC, you get the jolt.

I'm not surprised that the cap would be isolated when the function is changed.    It's too bad the manufactures no longer supply schematics.   

[kawal]

Joe
Good video - Didn't realize you can  get a  meter to damage your equipment or give you a shock.  Very informative. 

Kawal

[Fungus]

Good video - Didn't realize you can  get a  meter to damage your equipment or give you a shock.

Don't worry, I don't think anybody else did either.

(and we're all paranoid now...?)

[joeqsmith]

Let's summarize (again):
1. The OP found a way to "fool" his shiny new Brymen BM789 when set to the DC-coupled high-resolution ACmV range. This basically involves saturating the input circuitry such that the DMM cannot measure the (small) AC component because of a (large) DC bias. Under these conditions the DMM neither detects an overload condition nor provides a correct measurement of the (small) AC component.
2. joeqsmith has demonstrated that a number of different DMMs from different brands can be "fooled" using a similar method.

I think the lesson here, if there is one, is that using a DMM  (any model from any brand) to get correct measurements always involves a minimum of thinking - as 2N3055 reminded us. As well as spending a few seconds to read and understand the User Manual.

Edit: Sorry 2N3055, we seem to have posted our summaries almost simultaneously. I defer to yours which is much better than mine.

Edit2: I can think of a number of reasons why Brymen did not include a series capacitor in the ACmV range front end, and similarly for other DMMs from other brands. But that would be a matter for (reasoned) discussion in another thread, if you ever want to. I for one would be very interested in your opinion.

Thank you for the kind words.
I would not mind if you started that discussion, but in my opinion it would necessitate at least partial R.E of front end and switching circuit. Otherwise we would all have many ideas, but no confirmation. I guess Joe did quite some work on that already..
It might even be that meter could be bodged with a capacitor as an afterthought. But I would be very reluctant to do that to my meter and also to publish it on Internet. I have same opinion on that as Joe has about his "hardenings" of various meters. Maybe that is a Pandora box better be left unopened.
Best,

I have made my rules about making comments on my YT channel and have filters help the signal to noise ratio.  The rules are of course in the very first post:
https://www.eevblog.com/forum/testgear/hear-kitty-kitty-kitty-nope-not-that-kind-of-cat/

Once in a while I still get a few gems from the kiddies.   Attached is a recent post that relates to your comment.  I have removed their name as they run a small channel and see no reason to advertise for them.

They are correct that people continue to ask about my specific mods, even though the rules are clear.  They are also correct that I will normally provide a totally dick response.    I'm amazed that after so many years anyone would expect a different outcome.   

I like the part about having millions of viewers (love that drama), the reality is we are normally talking about a few thousand.   That's already a far larger group than I would expect for such a niche subject.   
 
While the FAQ covers this subject and even includes a few of the earlier like minded comments, my comment about John D. Rockefeller giving stock advice is a case were an ignorant person like our friend here makes a similar request only to file a lawsuit is something people should read about.   From my perspective, there is also a question of being responsible.  The meters being a safety device should not be tampered with.  YouTube may also frown on a channel promoting unsafe behavior. 

The easiest thing for me to do would be to not make any content, but there's a few of you who seem to enjoy it.  So for now, I plan to continue my unfunded, sparse and low quality content.   

[2N3055]

Let's summarize (again):
1. The OP found a way to "fool" his shiny new Brymen BM789 when set to the DC-coupled high-resolution ACmV range. This basically involves saturating the input circuitry such that the DMM cannot measure the (small) AC component because of a (large) DC bias. Under these conditions the DMM neither detects an overload condition nor provides a correct measurement of the (small) AC component.
2. joeqsmith has demonstrated that a number of different DMMs from different brands can be "fooled" using a similar method.

I think the lesson here, if there is one, is that using a DMM  (any model from any brand) to get correct measurements always involves a minimum of thinking - as 2N3055 reminded us. As well as spending a few seconds to read and understand the User Manual.

Edit: Sorry 2N3055, we seem to have posted our summaries almost simultaneously. I defer to yours which is much better than mine.

Edit2: I can think of a number of reasons why Brymen did not include a series capacitor in the ACmV range front end, and similarly for other DMMs from other brands. But that would be a matter for (reasoned) discussion in another thread, if you ever want to. I for one would be very interested in your opinion.

Thank you for the kind words.
I would not mind if you started that discussion, but in my opinion it would necessitate at least partial R.E of front end and switching circuit. Otherwise we would all have many ideas, but no confirmation. I guess Joe did quite some work on that already..
It might even be that meter could be bodged with a capacitor as an afterthought. But I would be very reluctant to do that to my meter and also to publish it on Internet. I have same opinion on that as Joe has about his "hardenings" of various meters. Maybe that is a Pandora box better be left unopened.
Best,

I have made my rules about making comments on my YT channel and have filters help the signal to noise ratio.  The rules are of course in the very first post:
https://www.eevblog.com/forum/testgear/hear-kitty-kitty-kitty-nope-not-that-kind-of-cat/

Once in a while I still get a few gems from the kiddies.   Attached is a recent post that relates to your comment.  I have removed their name as they run a small channel and see no reason to advertise for them.

They are correct that people continue to ask about my specific mods, even though the rules are clear.  They are also correct that I will normally provide a totally dick response.    I'm amazed that after so many years anyone would expect a different outcome.   

I like the part about having millions of viewers (love that drama), the reality is we are normally talking about a few thousand.   That's already a far larger group than I would expect for such a niche subject.   
 
While the FAQ covers this subject and even includes a few of the earlier like minded comments, my comment about John D. Rockefeller giving stock advice is a case were an ignorant person like our friend here makes a similar request only to file a lawsuit is something people should read about.   From my perspective, there is also a question of being responsible.  The meters being a safety device should not be tampered with.  YouTube may also frown on a channel promoting unsafe behavior. 

The easiest thing for me to do would be to not make any content, but there's a few of you who seem to enjoy it.  So for now, I plan to continue my unfunded, sparse and low quality content.

Oh, THAT guy...  hehehe well he doesn't like you, wonder why ...

Problem is, and nobody wants to hear this, is that you gave enough information to point out in a right direction what you think problem is.. For people that understand what is going on. They have enough info to experiment themselves.
Those that don't know enough to pick up the hints are probably not qualified enough to poke around inside meter.
Even if you publish the schematic and clear photos what needs to be done, will they follow the instructions to the letter?
Will they use same MOVs, PTCs, surge rated resistors, same Corona Dope, or Teflon inserts?
Add to that hordes of people that measure on the secondary of MOT, and then are baffled why the meter is smoking..
I wouldn't want to bear that responsibility. It's called social responsibility.
If people want to walk outside the line, it is their problem and responsibility. I don't want to be enabler.

We had same type of discussion some time ago, when people started promoting that people shouldn't buy cheap scopes from Rigol and Siglent because they figured out how 1000 EDU series from Keysight can be "hacked" and "you get real scope"..
There was heated discussion about it when it was pointed out that "hacking" Keysight scope involved physical removal of components and soldering on the motherboard that not only kills your warranty, but may lead to damaging brand new scope, that still cost more than some very nice scopes from competition. They were making it look simple, but it is not for beginners and people that don't have proper experience and equipment.
We still have few topics here on forum, where people are trying to repair Keysight scopes damaged in attempt to hack it.
Draw your own conclusions..

[AndrewBCN]

  It seems I got quoted? 

Actually 2N3055 this is what you wrote:

...
- Meters SHOULD AC couple in AC only measurements so you could measure 20mV AC riding on top of 100V
- For some reason Brymen doesn't do that on AC mV range. That is NOT good way to do it.
...

I believe Brymen does not AC couple the ACmV range because they don't want to have to deal with the kind of accident that Joe has clearly demonstrated can happen if you add a blocking capacitor in your DMM input circuitry, as Fluke did with the 189. It's simpler, safer and cheaper for a DMM manufacturer to just add a note in the User Manual that the meter will not provide a correct reading when the offset + signal > 1000mV, and make that range DC coupled, as Brymen does with most or all of their DMMs. And of course any user that knows a little bit what she/he is doing is going to use an external blocking capacitor when needed and be careful about discharging said capacitor after completing the measurement.

This is what I offered to discuss with you and others in a separate thread, but it seems you somehow misinterpreted my post as me wanting "to open Pandora's box". 

[2N3055]

It seems I got quoted? 

Actually 2N3055 this is what you wrote:

...
- Meters SHOULD AC couple in AC only measurements so you could measure 20mV AC riding on top of 100V
- For some reason Brymen doesn't do that on AC mV range. That is NOT good way to do it.
...

I believe Brymen does not AC couple the ACmV range because they don't want to have to deal with the kind of accident that Joe has clearly demonstrated can happen if you add a blocking capacitor in your DMM input circuitry, as Fluke did with the 189. It's simpler, safer and cheaper for a DMM manufacturer to just add a note in the User Manual that the meter will not provide a correct reading when the offset + signal > 1000mV, and make that range DC coupled, as Brymen does with most or all of their DMMs. And of course any user that knows a little bit what she/he is doing is going to use an external blocking capacitor when needed and be careful about discharging said capacitor after completing the measurement.

This is what I offered to discuss with you and others in a separate thread, but it seems you somehow misinterpreted my post as me wanting "to open Pandora's box". 

Yeah it's kind of misunderstanding or better say my prediction where that discussion would end up, not by you and me but maybe somebody else.

We cannot know whether Brymen did not include it because it was easy and lazy thing to do or there was intention and reason for it. Designer of the meter would know that, but we can only speculate. And in order to speculate on a level of "educated guess" instead of "some punters are crapshooting ideas" a reverse engineering could be done on meter inputs, and then you could see if decision to DC couple it had some simplification benefits (like less contacts on switch used, or simplification of layout, or whatever). It might have been because switch routing was simpler and provided more isolation distance or whatever.. Now I'm just throwing random ideas and that is useless..

Once reverse engineering of schematics was done, and alternative version that include capacitor was posited, there is a thin line between that and people that start cutting the board and adding capacitor into what is now a improvisation that outside looks  like a CAT IV meter and inside is a death trap..
I don't want to be complicit in something like that, and I personally don't care why. It is what it is, I use external cap if I need it, or lately just use MTX3293 if I need AC coupled mV. That one has the capacitor. But, mostly it is not an issue. I already spent too much time on it, and that only because I realized it was topic that was unknown to many, to my surprise. So it was good deed to spread the word, we also spoke a bit about good measurement practice (also good topic) and that's it.

If I ever decide to design my own meter design (hypothetically speaking, no intention to do so..) I would then think about it in more detail. I have no interest for it now.

[bdunham7]

I believe Brymen does not AC couple the ACmV range because they don't want to have to deal with the kind of accident that Joe has clearly demonstrated can happen if you add a blocking capacitor in your DMM input circuitry, as Fluke did with the 189. It's simpler, safer and cheaper for a DMM manufacturer to just add a note in the User Manual that the meter will not provide a correct reading when the offset + signal > 1000mV, and make that range DC coupled, as Brymen does with most or all of their DMMs. And of course any user that knows a little bit what she/he is doing is going to use an external blocking capacitor when needed and be careful about discharging said capacitor after completing the measurement.

I usually try to use logic and facts in these discussions, rather than characterization and hyperbole.  However, in your case I'm going to make an exception.

Your entire statement is an embarrassing fanboy apologist fantasy that exceeds, or a least meets, the highest levels of marketing spin that I've ever encountered.  The simplest explanation is that Brymen added mVAC to a range that would otherwise only be mVDC and mVDC+AC because they could do it at virtually zero additional expense using only software.

[bdunham7]

I would hate to think I left a few of you with the idea the having a meter show zero volts while in it's ACmV mode is unique to Brymen's new BM789,  here is another one for the UNI-T fan boys.   

Of course, switch it over to tri-display mode and we get a feel for what's really going on. 

I don't have any of the meters like the 789 to test, otherwise I would check myself--but has anyone confirmed that the DC overload issue is strictly confined to the dedicated mVAC range?  So the meters will all show 1VAC with 900VDC bias on the 6.0000V (or whatever is appropriate) range?

[2N3055]

I would hate to think I left a few of you with the idea the having a meter show zero volts while in it's ACmV mode is unique to Brymen's new BM789,  here is another one for the UNI-T fan boys.   

Of course, switch it over to tri-display mode and we get a feel for what's really going on. 

I don't have any of the meters like the 789 to test, otherwise I would check myself--but has anyone confirmed that the DC overload issue is strictly confined to the dedicated mVAC range?  So the meters will all show 1VAC with 900VDC bias on the 6.0000V (or whatever is appropriate) range?

I keep repeating. Yes it will. AC volts is AC coupled.

[floobydust]

For the Brymen and other's using the same chip, you wouldn't have a DC blocking cap. It can't go after the divider chain because well, it's inside the DMM IC. Outside the IC means it's on the hot side side of the divider chain, so a large expensive HV film cap plus another rotary switch contact is required, which costs more.

The real issue is the DMM IC saturating if DC is present and it's not reliably indicated by the firmware.  The op-amp in and out are available to the ADC, if the firmware looked.

[bdunham7]

For the Brymen and other's using the same chip, you wouldn't have a DC blocking cap. It can't go after the divider chain because well, it's inside the DMM IC. Outside the IC means it's on the hot side side of the divider chain, so a large expensive HV film cap plus another rotary switch contact is required, which costs more.

I keep repeating. Yes it will. AC volts is AC coupled.

I'm sorry if this seems repetitive or if I've missed something, but I'm not clear on what 'AC coupled' may mean in this case, I haven't seen anyone actually apply the test that I mentioned and in teardowns, I have not seen the HV blocking cap that I would expect.  So perhaps I'm not clear on some aspect of how the meter works.  I'm willing to believe 2N3055 is correct and I'm wrong, but I would like to know where I'm wrong.

First, I'll assume that like most of these, the AC input is actually not totally AC coupled and will in fact have a measured 10-11M input resistance when in the AC mode and measured with another meter.  If that isn't true, then that's where I'm wrong.

Second, I'll assume that indeed my observation that there is no 1kV HV blocking cap is correct and there is not in fact one lurking in there somewhere.  Again, if there is one, that's where I'm wrong.

Third, if the first two are true, then I'll assume that the AC coupling is added somewhere after the voltage divider and if that is a capacitor, it is a smaller, lower voltage variety which is only exposed to the voltage after the divider...but those dividers don't really work in such a way as to make it easy to shove a capacitor in there, do they?   So what does that look like?   And on the 6.0000V range, what would such a blocking capacitor be exposed to if the input voltage was 5VAC + 600VDC?

Just to be thorough, I tested my F116 with 5VAC + 100 to 600VDC.  Each step up resulted in a brief 'OL', then back to a 5VAC reading.  I took it apart and there appears to be a rectangular capacitor C1 near the switch that could plausibly be a blocking capacitor, although it is a bit smaller than others I've seen. 

[Caliaxy]

I don't have any of the meters like the 789 to test, otherwise I would check myself--but has anyone confirmed that the DC overload issue is strictly confined to the dedicated mVAC range?  So the meters will all show 1VAC with 900VDC bias on the 6.0000V (or whatever is appropriate) range?

I keep repeating. Yes it will. AC volts is AC coupled.

Right – the issue solely occurs on the "mV" position, in AC mode. The rotary dial "mV" position has 6 modes you can circle through by pressing the yellow soft “Select” button:

DC mV
AC mv
AC+DC mV (a combined RMS value)
frequency
duty cycle
dBm*

Interestingly, the “AC mV” position is the only one in which the meter can’t figure it’s overloaded (in the presence of a DC bias) and displays a false value. “DC mV” and “AC+DC mV” show, correctly “OL”.

As circling through these 6 modes doesn’t require moving the rotary dial, there is hope that the issue could be, in theory, fixed in software (by “fixing” I mean having the meter display “OL”, as it correctly does in "AC+DC mV" mode**). It won’t help the current owners, because the firmware is not user upgreadable, but still (it will stop people complaining about this issue and bring this thread to an end; not sure about the latter ).

*dBm also doesn’t show “OL” when it should, but that’s just a calculated value based on whatever the meter thinks it measures in AC.
**haven't checked extensively (and won't...), but the AC+DC mV mode seems to take longer to display OL when AC+DC overloaded, and it sometimes displays a first wrong value which is quickly updated to OL, suggesting that it does some extra measurements/calculations. Whatever it does, maybe it could be done in AC mV mode as well?

[floobydust]

Just look at the schematics, and for the big 1,000V film cap at the input to the divider string. Looking at a few:
34401a AC-coupled C301 0.22uF 400V max DC bias.
34410a AC-coupled C400 0.22uF 400V max DC bias.
3478a  AC-coupled C301 0.15uF 630V
Agilent U1273a AC-coupled C30 0.1uF 630V
Fluke 101 AC-coupled 0.01uF 1,000V C1
Fluke 189 AC-coupled 0.01uF 1,000V
Fluke 29/79II, III Fluke 87 old; AC-coupled 0.01uF 1,000V C3
Fluke 87V; AC-coupled 0.022uF 1,000V C1
Fluke 867; AC-coupled 0.1uF 1,000V C31
Fluke 27 AC-coupled 0.022uF 1,000V C3
Fluke 10/11/12 AC-coupled 0.01uF 1,000V C14
Fluke 17B DC-coupled
AN8008 DC-coupled
UT-61e DC-coupled

For the hardware to have a DC-blocking cap, if the CMOS switches and mV AC op-amp and true-RMS converter are all in the DMM IC, then you simply can't AC-couple post-divider without something getting swamped. Like the ANENG AN8008, Brymen 789, 121GW, Fluke 17B, UT61e. These all can get overloaded from the DC.
The issue is if these detect and display it.

Note the true-RMS DSP also loses the DC component, as well as an RMS converter IC i.e. AD637. So this is a second (downstream) location in the chain where the DC component is removed. BUT everything ahead of this can get saturated- unless there's a blocking cap at the input of the divider chain which the more expensive multimeters incorporate.

[joeqsmith]

I would hate to think I left a few of you with the idea the having a meter show zero volts while in it's ACmV mode is unique to Brymen's new BM789,  here is another one for the UNI-T fan boys.   

Of course, switch it over to tri-display mode and we get a feel for what's really going on. 

I don't have any of the meters like the 789 to test, otherwise I would check myself--but has anyone confirmed that the DC overload issue is strictly confined to the dedicated mVAC range?  So the meters will all show 1VAC with 900VDC bias on the 6.0000V (or whatever is appropriate) range?

So we want to overdrive the ACV input with high DC offset?  So we put about 2KVAC into the meter and adjust the offset by a KVDC?   Seems like fun.  I'll sit back and watch this one. 

If you're interested in more details on the 189, I took a quick stab at tracing out this area.   I also made an attempt to look at the current into a short. 

[bdunham7]

Just look at the schematics,

I have or have seen the schematics for the first 8 you listed, but I don't have and don't recall the details of any DC-coupled models.  If you have a 10M input resistor and can live with a 30-40Hz cutoff, the capacitor can be pretty small.

For the hardware to have a DC-blocking cap, if the CMOS switches and mV AC op-amp and true-RMS converter are all in the DMM IC, then you simply can't AC-couple post-divider without something getting swamped. Like the ANENG AN8008, Brymen 789, 121GW, Fluke 17B, UT61e. These all can get overloaded from the DC.
The issue is if these detect and display it.

They don't in mVAC, but I'm wondering if/how they cope with the issue in the higher ACV ranges.  And if they indeed use a pre-divider HV cap, then not incorporating that in front of the mVAC range has to be for other reasons. 

[bdunham7]

So we want to overdrive the ACV input with high DC offset?  So we put about 2KVAC into the meter and adjust the offset by a KVDC?   Seems like fun.  I'll sit back and watch this one. 

No, not quite that high!  I want to use the lowest 'AC coupled' non-mVAC range with an appropriate in-range AC signal and add DC bias much larger than the range but not outside the meters stated limitations.  So, Fluke 116, 5VAC on 6.000VAC range with 600VDC bias--works fine.  Brymen 789 on 6.0000VAC range with 5VAC + 1kV DC bias (or whatever is handy), works .  And is there an HV cap in there somewhere?

[Caliaxy]

First, I'll assume that like most of these, the AC input is actually not totally AC coupled and will in fact have a measured 10-11M input resistance when in the AC mode and measured with another meter.

The DC V mode (which allows combined DC+AC rms measurments) and the mV mode (AC, DC and AC+DC) on BM789 present, indeed a 10-11 MOhms input resistance (as measured by 189). The plain AC V mode shows high impedance (OL, above what 189 can measure).

[joeqsmith]

So we want to overdrive the ACV input with high DC offset?  So we put about 2KVAC into the meter and adjust the offset by a KVDC?   Seems like fun.  I'll sit back and watch this one. 

No, not quite that high!  I want to use the lowest 'AC coupled' non-mVAC range with an appropriate in-range AC signal and add DC bias much larger than the range but not outside the meters stated limitations.  So, Fluke 116, 5VAC on 6.000VAC range with 600VDC bias--works fine.  Brymen 789 on 6.0000VAC range with 5VAC + 1kV DC bias (or whatever is handy), works .  And is there an HV cap in there somewhere?

Where's the fun in that?  As I previously mentioned, a kV is a bit too risky (not the meter but TE).   But, if you will settle for something a bit lower, shown is 6.5VAC in the lowest ACV range (manual) with a -600VDC offset and again at 700VDC. 

[AndrewBCN]

...
Yeah it's kind of misunderstanding or better say my prediction where that discussion would end up, not by you and me but maybe somebody else.

We cannot know whether Brymen did not include it because it was easy and lazy thing to do or there was intention and reason for it. Designer of the meter would know that, but we can only speculate. And in order to speculate on a level of "educated guess" instead of "some punters are crapshooting ideas" a reverse engineering could be done on meter inputs, and then you could see if decision to DC couple it had some simplification benefits (like less contacts on switch used, or simplification of layout, or whatever). It might have been because switch routing was simpler and provided more isolation distance or whatever.. Now I'm just throwing random ideas and that is useless..

Once reverse engineering of schematics was done, and alternative version that include capacitor was posited, there is a thin line between that and people that start cutting the board and adding capacitor into what is now a improvisation that outside looks  like a CAT IV meter and inside is a death trap..
I don't want to be complicit in something like that, and I personally don't care why. It is what it is, I use external cap if I need it, or lately just use MTX3293 if I need AC coupled mV. That one has the capacitor. But, mostly it is not an issue. I already spent too much time on it, and that only because I realized it was topic that was unknown to many, to my surprise. So it was good deed to spread the word, we also spoke a bit about good measurement practice (also good topic) and that's it.

If I ever decide to design my own meter design (hypothetically speaking, no intention to do so..) I would then think about it in more detail. I have no interest for it now.

Thanks for clearing that up, 2N3055 and I perfectly understand and agree with your position and Joe's about avoiding and not recommending modifications to the input circuitry of any instrument rated for high voltages. I fully subscribe to that.

I guess the discussion in this thread was sufficiently fruitful in the sense that it is clear (to me at least) that among the best practices for DMM usage, shorting the leads before and after any measurement can help avoid some kinds of accidents. And there is no need for further discussion of the input stages of various DMMs or positing about what the exact intentions of Brymen engineers were when they decided to make the input stage of the ACmV range of the BM789 DC-coupled - as you pointed out, we'll probably never know for sure.

[bdunham7]

The DC V mode (which allows combined DC+AC rms measurments) and the mV mode (AC, DC and AC+DC) on BM789 present, indeed a 10-11 MOhms input resistance (as measured by 189). The plain AC V mode shows high impedance (OL, above what 189 can measure).

Where's the fun in that?  As I previously mentioned, a kV is a bit too risky (not the meter but TE).   But, if you will settle for something a bit lower, shown is 6.5VAC in the lowest ACV range (manual) with a -600VDC offset and again at 700VDC.

Well, that was easy.  Looks like I'm wrong on the first count and there is an HV capacitor right in front somewhere and it does just what it should.  So the decision vis-a-vis the mVAC range, like the F116, isn't over the cost of a capacitor.

Of course it means that the BM789 also stores device-destroying energy in the AC mode! 

[floobydust]

[...] it is clear (to me at least) that among the best practices for DMM usage, shorting the leads before and after any measurement can help avoid some kinds of accidents.

NO!!! This is wrong, where did you pick this up from? If the equipment has a charged up DC-blocking cap, it will discharge into the front-end and kill it.

[AndrewBCN]

[...] it is clear (to me at least) that among the best practices for DMM usage, shorting the leads before and after any measurement can help avoid some kinds of accidents.

NO!!! This is wrong, where did you pick this up from? If the equipment has a charged up DC-blocking cap, it will discharge into the front-end and kill it.

 

How would a cap that is part of the front-end discharge into the front-end when the leads are shorted?

Joe, we need another video!!!! 

[floobydust]

Just look at the schematics,

I have or have seen the schematics for the first 8 you listed, but I don't have and don't recall the details of any DC-coupled models.  If you have a 10M input resistor and can live with a 30-40Hz cutoff, the capacitor can be pretty small.

For the hardware to have a DC-blocking cap, if the CMOS switches and mV AC op-amp and true-RMS converter are all in the DMM IC, then you simply can't AC-couple post-divider without something getting swamped. Like the ANENG AN8008, Brymen 789, 121GW, Fluke 17B, UT61e. These all can get overloaded from the DC.
The issue is if these detect and display it.

They don't in mVAC, but I'm wondering if/how they cope with the issue in the higher ACV ranges.  And if they indeed use a pre-divider HV cap, then not incorporating that in front of the mVAC range has to be for other reasons.

For the mV range, the input attenuator is not employed and (DC-coupled) the DMM IC gets the same voltage input as the test leads, almost directly (post-PTC) connected to the IC. There is a 10MEG load switched in.
There is next a mux and op-amp or PGA because you need a gain of ~4-10x before the ADC to have a decent noise floor. 141mV in, X10 and you've got 1.41V going to the op-amp then ADC.
What happens with say 1V DC on 100mVAC ? The amp is saturated or even phase-inverts and you can get bogus readings. Unless you are constantly checking (in firmware) for this, which is what we should be checking for.
For AC-coupled equipment, discharging the blocking cap into the front-end, say 200V and a few hundred K seems to be at most a few mA but JFET, CMOS mux etc. don't always survive. It seems to be the high voltage damaging the semi. Some gear does not have much for series resistance or a huge cap and the discharge hits hard.

[Fungus]

If you're interested in more details on the 189, I took a quick stab at tracing out this area.   I also made an attempt to look at the current into a short. 

Have you painted your 189?

[Fungus]

NO!!! This is wrong, where did you pick this up from? If the equipment has a charged up DC-blocking cap, it will discharge into the front-end and kill it.

Doesn't it have input protection for that? 

[2N3055]

For the Brymen and other's using the same chip, you wouldn't have a DC blocking cap. It can't go after the divider chain because well, it's inside the DMM IC. Outside the IC means it's on the hot side side of the divider chain, so a large expensive HV film cap plus another rotary switch contact is required, which costs more.

I keep repeating. Yes it will. AC volts is AC coupled.

I'm sorry if this seems repetitive or if I've missed something, but I'm not clear on what 'AC coupled' may mean in this case, I haven't seen anyone actually apply the test that I mentioned and in teardowns, I have not seen the HV blocking cap that I would expect.  So perhaps I'm not clear on some aspect of how the meter works.  I'm willing to believe 2N3055 is correct and I'm wrong, but I would like to know where I'm wrong.

Well if you put, like I suggested (also repetitively) other meter in set in ohms to input of the meter in question, you will see if input impedance going high, to isolation range. Also, you can take a meter in question, and exactly what was said: you charge meter input wit, say 12V to be safe and discharge it into scope probe set to safe range. If you see exponential decay, there you are you have capacitance.

Also, there is only one way to do it, really. So AC coupled, in this context, means serial capacitor...

[joeqsmith]

If you're interested in more details on the 189, I took a quick stab at tracing out this area.   I also made an attempt to look at the current into a short. 

Have you painted your 189?

This was that meter that I bought to transient test.  You will find my posts on painting it here:

https://www.eevblog.com/forum/testgear/painting-over-rubber/msg1827791/#msg1827791

Over the years, the paint peeled off and you are seeing the primer.  The rubberized paints just don't seem to adhere well in this application.   

[joeqsmith]

[...] it is clear (to me at least) that among the best practices for DMM usage, shorting the leads before and after any measurement can help avoid some kinds of accidents.

NO!!! This is wrong, where did you pick this up from? If the equipment has a charged up DC-blocking cap, it will discharge into the front-end and kill it.

 

How would a cap that is part of the front-end discharge into the front-end when the leads are shorted?

Joe, we need another video!!!! 

The last video shows me shorting the leads after applying a kV to meter in an attempt to look at the current.   Maybe that's how they design them in Canada.  floobydust would need to provide a brand/model for one that can be damaged as they suggest.      If I have the meter, I would certainly be willing to attempt to replicate their findings.     

[joeqsmith]

Well, that was easy.  ...

Of course it means that the BM789 also stores device-destroying energy in the AC mode! 

I could run it up higher if you wanted but would want to make a few changed to my setup just to play it safe.   

Yes, after applying a DC voltage to the 789 while it is in it's ACV mode, I can then discharge the meter to the LED bank and have it light.  I would suggest it stores potentially device-distroying energy.   The parasitics alone may be enough to damage something sensitive.  If you watched that last video, that was basically my point.  This isn't a static problem in the sense that everything is at steady state.   Measuring the DC resistance has little to do with the potential peak currents.   

[Fungus]

The last video shows me shorting the leads after applying a kV to meter in an attempt to look at the current.   Maybe that's how they design them in Canada.  floobydust would need to provide a brand/model for one that can be damaged as they suggest.      If I have the meter, I would certainly be willing to attempt to replicate their findings.   

Surely the stress of discharging it is no worse than the stress of charging it.

(assuming a low source impedence for charging).

[joeqsmith]

The last video shows me shorting the leads after applying a kV to meter in an attempt to look at the current.   Maybe that's how they design them in Canada.  floobydust would need to provide a brand/model for one that can be damaged as they suggest.      If I have the meter, I would certainly be willing to attempt to replicate their findings.   

Surely the stress of discharging it is no worse than the stress of charging it.

(assuming a low source impedence for charging).

Considering the attached schematic, assuming V+ is 12V, D is a silicon diode,  R is 10kohms, C is 10uF and all components are perfect (no parasitics,  switch resistances are zero when closed and infinite when open), under what condition will the current through the capacitor reach it's highest peak current?

A) when charging
B) when discharging
C) A and C
D) non of the above
E) what's a diode?

[Fungus]

Considering the attached schematic, assuming V+ is 12V, D is a silicon diode,  R is 10kohms, C is 10uF and all components are perfect (no parasitics,  switch resistances are zero when closed and infinite when open), under what condition will the current through the capacitor reach it's highest peak current?

Does current flow through a capacitor? 

A) when charging
B) when discharging
C) A and C
D) non of the above
E) what's a diode?

Assuming you meant the peak current flowing at point "I" in the circuit, I'm gonna go with (A)

[joeqsmith]

Considering the attached schematic, assuming V+ is 12V, D is a silicon diode,  R is 10kohms, C is 10uF and all components are perfect (no parasitics,  switch resistances are zero when closed and infinite when open), under what condition will the current through the capacitor reach it's highest peak current?

Does current flow through a capacitor? 

A) when charging
B) when discharging
C) A and C
D) non of the above
E) what's a diode?

Assuming you meant the peak current flowing at point "I" in the circuit, I'm gonna go with (A)

First hit on displacement current gives us wiki:
https://en.wikipedia.org/wiki/Displacement_current

For the meters, I would assume that the charge/discharge would be basically the same.  Even on the 189, it looks like it follows two different paths.  Still the difference is  negligible.   Still, it's common to find exceptions when making blanket statements. 

[Fungus]

For the meters, I would assume that the charge/discharge would be basically the same.  Even on the 189, it looks like it follows two different paths.  Still the difference is  negligible.   Still, it's common to find exceptions when making blanket statements.

It's hard to imagine a measuring device that would want the capacitor to discharge faster than it charges. It would go horribly wrong as the AC frequency increases.

[bdunham7]

Well if you put, like I suggested (also repetitively) other meter in set in ohms to input of the meter in question, you will see if input impedance going high, to isolation range. Also, you can take a meter in question, and exactly what was said: you charge meter input wit, say 12V to be safe and discharge it into scope probe set to safe range. If you see exponential decay, there you are you have capacitance.

Also, there is only one way to do it, really. So AC coupled, in this context, means serial capacitor...

I don't have this meter to test, so until Caliaxy and joeqsmith posted, I had no way of knowing what this result would be.

There is not only one way to do it, this I know because I have examples where it was not done in exactly the same way.  The F116, for example, passes the HV blocking test even on the lowest (non-mV) range of 6.000V.  However, its measured input resistance (DC) is 10M on all ranges.  This is an issue of some significance to me because I'm using HV divider probes to measure HV at times and I've always wanted better results.  Depending on the divider probe, you could end up charging a blocking cap to the full HVDC bias, which would be bad--so you would avoid using a truly AC-coupled range with an HV probe that didn't have a lower divider resistor.  Fortunately most of them DO have  this lower divider, but I did have one go open. 

So I'm still learning tidbits here and there that I haven't thought of.  The F116 is fairly new to me, and I hadn't considered the input characteristics beyond knowing that the AC input had a 10M straight resistance across it.  Only with this thread did I discover that the mVAC range is actually only mVAC+DC.  Now I know that the F116 also eliminates the hazards of a charged AC blocking capacitor, since the 10M across the front end will quickly discharge it.  I'm not saying that Fluke did this deliberately to prevent HVAC technicians from frying control circuitry, since that might be an " embarrassing fanboy apologist fantasy", but it is a characteristic.  Interestingly, they seem to have done this by using a 10M capacitive coupled input in parallel with a 10M resistor across the front end, giving the meter an effective input impedance of 5M on the AC ranges.  That does actually seem deliberate.   And it messes up my 80k-40 HV probe by 10% or so...

[joeqsmith]

Well, that was easy.  ...

Of course it means that the BM789 also stores device-destroying energy in the AC mode! 

I could run it up higher if you wanted but would want to make a few changed to my setup just to play it safe.   

Yes, after applying a DC voltage to the 789 while it is in it's ACV mode, I can then discharge the meter to the LED bank and have it light.  I would suggest it stores potentially device-distroying energy.   The parasitics alone may be enough to damage something sensitive.  If you watched that last video, that was basically my point.  This isn't a static problem in the sense that everything is at steady state.   Measuring the DC resistance has little to do with the potential peak currents.

Shown with the 789 set to the ACV mode, again with it set to the lowest manual range.   Fluke 189 will over range at 1100V.  Shown at 1050VDC offset then again with 1200VDC.   

[joeqsmith]

For the meters, I would assume that the charge/discharge would be basically the same.  Even on the 189, it looks like it follows two different paths.  Still the difference is  negligible.   Still, it's common to find exceptions when making blanket statements.

It's hard to imagine a measuring device that would want the capacitor to discharge faster than it charges. It would go horribly wrong as the AC frequency increases.

Say you wanted to make an oscillator out of CMOS gates and have something other than a  50% duty cycle.  Rather than a simple RC to set the timing, we could use this circuit to provide two different time constants.   Maybe your measuring device has an RC network on the CPUs reset for a slow start but when the power is cycled, you want it to reset fast.   It's not that uncommon. 

[Fungus]

Maybe your measuring device has an RC network on the CPUs reset for a slow start but when the power is cycled, you want it to reset fast.   It's not that uncommon.

That's a measurement device's reset circuit, not a measurement device's measuring circuit.

[joeqsmith]

Maybe your measuring device has an RC network on the CPUs reset for a slow start but when the power is cycled, you want it to reset fast.   It's not that uncommon.

That's a measurement device's reset circuit, not a measurement device's measuring circuit.

Your comment had broadened the scope. 

It's hard to imagine a measuring device that would want the capacitor to discharge faster than it charges.

[floobydust]

NO!!! This is wrong, where did you pick this up from? If the equipment has a charged up DC-blocking cap, it will discharge into the front-end and kill it.

Doesn't it have input protection for that? 

Summarizing, the BM789s is DC-coupled as far as the hardware and DMM IC, and there's a HPF from the true-RMS DSP section if Brymen is using it verses the MCU doing the math with raw A/D samples.
So I believe it can be saturated with DC on any ACV measurements if it can't auto-range up, and the F/W needs a check to see of if it lets you know this is happening.

I looked at the usual dozen multimeter schematics, the ones incorporating a DC-blocking cap for ACV. Maybe others can contribute.
None of the asian design multimeters I can find are using a DC-blocking cap, except the BM869s.
The "protection circuit" is needed to clamp down to the IC level, the CMOS switches downstream of the HV protection MOV's. Interestingly, the AC path does not include anything like our usual diode-connected transistors etc. which end up on the DC/ohms path. AC protection is by large series resistance, 87V is 10MEG and 34401a is 1MEG so the cap discharge is low current, enough for an IC's substrate diodes to handle. But the voltage is still high for whatever you are probing, not sure what a mosfet does with HV on the gate at low current, for a long time. ESD is short duration and 10X and the voltage at 1/20 to 1/200 the capacitance.

Interestingly ACV the 87V moves the 9.99MEG after the cap, so it's a 10MEG AC impedance and very high (leakage) DC resistance.
So using a single-resistor HV probe with the DMM on ACV would charge to a clamped couple kV DC-component and not be a good experience.

Oscilloscopes have larger blocking cap (up to 22X) and lower impedances for the bandwidth, some using relays/switches or maybe solid-state AC/DC coupling switching. The cap discharge is brutal there maybe 15mJ across the switch and goodbye JFET if you short the probe's input. 2235a rated 400VDC goes 0.022uF down to 1,000pF +ve discharge through 70R to the JFET, minus a 10X probe. It all depends on the moves you make while the cap has a HV charge.

As far as providing a make/model/schematic a showman could use, awareness of the charged cap is more important than damaging something. I treat it like a grenade- a high risk measurement (AC ripple/signal on HV DC) across the board.

[bdunham7]

None of the asian design multimeters I can find are using a DC-blocking cap, except the BM869s.

My BK 391A definitely had a blocking cap and measured infinite DC impedance on ACV.

According to other tests here, the BM 78x series has a DC blocking cap as well for ACV above the mVAC range.

[floobydust]

BM789 - I don't see a DC blocking cap in any PC board photos.

[bdunham7]

BM789 - I don't see a DC blocking cap in any PC board photos.

You have to give the Dave credit for posting photos of the guts--not too many manufacturers do that anymore.

I also had a hard time with that, although I think it is likely to be the lower of the two components I have rectangled here:

[joeqsmith]

Look close and you may notice the part designator for the larger component shows R.  I believe this is one of the shunts but to be clear, I have not spent any time trying to trace it out. 

[bdunham7]

Look close and you may notice the part designator for the larger component shows R.  I believe this is one of the shunts but to be clear, I have not spent any time trying to trace it out.

I did see that designator originally, but without one in hand its hard to know if it means the white rectangular thing that looks a lot like a capacitor or not.  I don't know where else it could be except perhaps up masquerading as a PTC.  I doubt they'd let high voltage get up north of that switch anywhere.  I do see a C1 on the back that looks to be in the right spot, but it looks too small?  Perhaps SMT HV capacitors are more advanced than I know.

[floobydust]

At the bottom it's shunt R49 plate resistor MPC 72 or 73 I can't see all the text.
At the top box, the film cap looks small and one end connects directly to the DMM IC analog switches, so I think it's a filter that gets switched in/out maybe for VFD's. It's near the low voltage business on the board.
You can discern a bit from the switch video too https://www.eevblog.com/forum/testgear/bm786-switch-issue/msg3515584/#msg3515584

I'll follow traces on C1, for DC-block it needs to connect to the rotary switch which is miles away it does at S1A1, S2A1 but not shorted out for the majority of functions like DCV, ohms etc. so I'm still skeptical.

The user manual mentions for 600mVAC "Signal peak absolute values, including DC bias, less than 1000mV peak" which I take as a 1V max. so no cap.

[bdunham7]

[ Specified attachment is not available ]

I'll follow traces on C1, for DC-block it needs to connect to the rotary switch which is miles away it does as S1A1, S2A1 but not shorted out for the majority of functions like DCV, ohms etc.

Actually it looks to me that C1 is within the borders of the switch cover, above the + jack and to the left of the PTCs.  I tried looking at Dave's videos regarding the switch problem where he has it torn apart, but they were mostly too closeup.  I'm betting C1 is it.

Edit:  C1 is right under where the switch contacts for the ACV range would be.  R49 is in fact a 1R resistor. I'm guessing the reason for the combined mV system is that they were just out of switch positions due to the design not being 360-degree capable

[joeqsmith]

I believe that's it but again, have not looked.   I gave a little demonstration on using a 100 year old blow torch to do some surface mount soldering and had pulled this capacitor from the board.  It ended up stuck on the one pad.  It would be easy enough to measure the value and trace it out. 

10000pF at a kV+ seems doable in this package. 
https://product.tdk.com/en/system/files?file=dam/doc/product/capacitor/ceramic/mlcc/catalog/mlcc_commercial_highvoltage_en.pdf

[Neutrion]

OK so we now know that there is at least a positive aspect of the DC coupling, so if Brymen could manage to adress the "overrange problem", it could be safer on the mV scale that some other meters.

I was trying to characterize the meter to know above which level I can not trust it anymore on the AC mV scale. In the worst case scenario when the width of the squarewave is just above the declared minimum of 350us this safety limit can be as low as 400-430mV. Above which the meter starts to show BS values. So it is not necessarily has to reach the 650mv displayed value to have something way higher real value. Even with 50% duty cycle It starts to show invalid values above 500mV.

Unfortunately we still don't know what happens, if we just raise the voltage to lets say 50V. Maybe it will overrange. What is this lowest safety marging with the 869?
The other question is, what could be the lowest safety margin with some kind of completely  different waveform? Could it go even further down? Would be really nice to know.

Anyone has a newer firmware than the 08?

So can I can get the BM789 to read 0 mVAC with a signal applied as I suggested.  You bet.  Do I care, other than for a small bit of entertainment, not at all.        I could run others but will wait and see what bdunham7 comes up with.

What was the exact waveform here?


And regarding the AC coupled mode:

Similar to the ipact wrench, or cordless drill tests where they are getting attached to eachother to see which is the stronger, we might see multimeter fights as well in the future!


It is really interesting that this issue surprises even experienced engineers. Might there are tonns of equipment which are getting damaged all the time because of this regulary, just people are not avare of the cause. Wonder how long they keep the charge.

[2N3055]

It is really interesting that this issue surprises even experienced engineers. Might there are tonns of equipment which are getting damaged all the time because of this regulary, just people are not avare of the cause. Wonder how long they keep the charge.

I don't find it interesting if experienced engineers would be surprised by this.
Thing that would come to my mind would be more like: they should know better.....

Don't get me wrong, I'm not talking about making mistakes. But if experienced engineer doesn't understand this, maybe he/she is not as experienced as they think. At least not in measurement and measurement instruments basics..

[bdunham7]

I don't find it interesting if experienced engineers would be surprised by this.
Thing that would come to my mind would be more like: they should know better....

If they hadn't run into it or thought of it, well that's me and a lot of other people.  If they don't understand it once tested and explained, that's different.  I would have thought that if it was an issue, the DMM manufacturers would take steps to alleviate it--which is completely straightforward and doable-- but apparently they haven't in all cases.  You do need a fairly unusual series of steps to make all that happen, so I doubt it is a regular occurrence.

[Fungus]

Hands on hearts:

How many people knew about this and took precautions before reading this thread?

[floobydust]

After the oscilloscope AC/DC coupling switch went kabang and shot out sparks, I had to think WTF just happened and spend time figuring it out because I couldn't afford to damage precious gear.

With ACmV readings, it's the situation where your measurement is out to lunch and you have to dig to find out why.
Is the test equipment damaged, or buggy software, used incorrectly, or a misunderstanding about what it can do? A lazy engineer will just sluff it off.

[joeqsmith]

So can I can get the BM789 to read 0 mVAC with a signal applied as I suggested.  You bet.  Do I care, other than for a small bit of entertainment, not at all.        I could run others but will wait and see what bdunham7 comes up with.

What was the exact waveform here?

I have no way to know exact.   From attached, it was roughly a 0.649 VRMS sine with a 2.84VDC bias.

And regarding the AC coupled mode:

Similar to the ipact wrench, or cordless drill tests where they are getting attached to eachother to see which is the stronger, we might see multimeter fights as well in the future!

I doubt it as the meters themselves should easily handle it. 

[Neutrion]

Hands on hearts:

How many people knew about this and took precautions before reading this thread?

I had no idea, but than I am also not an experienced engineer. I think with a lots of motors everywhere using inverters, this problem will pose more problems in the future than before.

So can I can get the BM789 to read 0 mVAC with a signal applied as I suggested.  You bet.  Do I care, other than for a small bit of entertainment, not at all.        I could run others but will wait and see what bdunham7 comes up with.

What was the exact waveform here?

I have no way to know exact.   From attached, it was roughly a 0.649 VRMS sine with a 2.84VDC bias.

Just because the problem is here generally the opposite,(showing something which is much less than it is) so I was wondering whether the sinewave might got too high frequency.
Because if not, than it means that the mentioned lowest safety limit could be as low as 0 Volt... Than there is no way of knowing whether because of DC bias an overrange situation occured.

With my little offset squarewave, below 400mV AC displayed value one can be on the safe side, but it seems that it is not the case with other waveforms. Like some small AC riding on some DC offset.

[2N3055]

I had no idea, but than I am also not an experienced engineer. I think with a lots of motors everywhere using inverters, this problem will pose more problems in the future than before.

Well, I don't think measuring milivolts and measuring VFD connected equipment have anything in common.

[Neutrion]

Charging up the meter tipically occurs at V AC in Brymen and both mV and VAC in Fluke it seems. So I was not talking about the missing overrange indication on the Brymen. 

[joeqsmith]

... so I was wondering whether the sinewave might got too high frequency.

All of those were done using a 60Hz sinewave.   

I had no idea, but than I am also not an experienced engineer. I think with a lots of motors everywhere using inverters, this problem will pose more problems in the future than before.

Well, I don't think measuring milivolts and measuring VFD connected equipment have anything in common.

I've never really given it any thought.  When I use an external block, out of habit I will discharge it.  When working above 24V I tend to take more precautions.

Much of the time when I need a meter, I am not using the AC modes.  Normally I'm not working on a mix of technologies at the same time where the internal block would even come into play.   If I am troubleshooting power electronics, I normally start with it unpowered.  When I get to where I am starting to bring it up, I am using a scope.   For logic, I'm using PC/JTAG, scope, LA.   

So, I doubt I will go out of my way to change any habits but it's certainly something to be aware of.   

Now, could Dave's 121GW diode test take out a sensitive device?   

[2N3055]

... so I was wondering whether the sinewave might got too high frequency.

All of those were done using a 60Hz sinewave.   

I had no idea, but than I am also not an experienced engineer. I think with a lots of motors everywhere using inverters, this problem will pose more problems in the future than before.

Well, I don't think measuring milivolts and measuring VFD connected equipment have anything in common.

I've never really given it any thought.  When I use an external block, out of habit I will discharge it.  When working above 24V I tend to take more precautions.

Much of the time when I need a meter, I am not using the AC modes.  Normally I'm not working on a mix of technologies at the same time where the internal block would even come into play.   If I am troubleshooting power electronics, I normally start with it unpowered.  When I get to where I am starting to bring it up, I am using a scope.   For logic, I'm using PC/JTAG, scope, LA.   

So, I doubt I will go out of my way to change any habits but it's certainly something to be aware of.   

Now, could Dave's 121GW diode test take out a sensitive device?

Agree..
Usual process is systematic, and there is a natural progression how one goes about troubleshooting things.

As for 121GW I don't know, but MTX3293 has 26V/10mA range in diode measurements.
I'm sure there are many sensitive devices that could be damaged by it.

[Neutrion]

But than also designing your own thing, and knowing all the guts of it (I suppose you Joe mostly design stuff) makes you to to take different measures than starting to measure around in something which you have not much idea about, maybe not even a schematics is aviable. And there could be also some hazardous failure modes, which in theory should not happen but they do sometimes.

Can any of you also bring the 869 to show 0 Volts with a dc offset? Though there with the dual display less of an issue.

[2N3055]

But than also designing your own thing, and knowing all the guts of it (I suppose you Joe mostly design stuff) makes you to to take different measures than starting to measure around in something which you have not much idea about, maybe not even a schematics is aviable. And there could be also some hazardous failure modes, which in theory should not happen but they do sometimes.

Can any of you also bring the 869 to show 0 Volts with a dc offset? Though there with the dual display less of an issue.

It is irrelevant if you are designing thing on your own, or you are repairing someone else's design.
If you are repairing something unknown, first order of business is to make it known, at least to the point to be able to understand where are the dangerous parts.  Any part of circuit that can become dangerous in any scenario (even the fault far outside normal operating parameters) should be permanently treated as dangerous.  Not to protect your meter but your life..

BM869 question I answered 3-4 times already. Yes and no.

[Neutrion]

I run through the topic, but only found this. But here you write about incorrect nubers, but not necessarily 0.

But does the 869 correctly shows "OL" for AC in this scenario?  Did you try to switch polarity?

With the 869  it makes sense to have everything on the same switch position, but with the single display if there is no way to show overrange they certainly could put in a separate AC mV position.

I think we have a language barrier here.

BM869S has 2 different positions for DC and AC mV, but both are nevertheless still DC coupled .

It does show OVL if you apply larger AC voltage. If you combine it with DC, that combined goes over range it also behaves funny.

For instance 1V P-P 400HZ square wave, will show correct 500mV RMS. If I add more than 0.6 V DC to it (offset) it will start showing wrong numbers. Funny enough, I can add -2,3V offset before it starts making problems. So it is not symmetric.

[joeqsmith]

But than also designing your own thing, and knowing all the guts of it (I suppose you Joe mostly design stuff) makes you to to take different measures than starting to measure around in something which you have not much idea about, maybe not even a schematics is aviable. And there could be also some hazardous failure modes, which in theory should not happen but they do sometimes.

Typically when I need to repair something that is new enough that I can't get schematics and it's actually something that is worth the time to attempt to repair, I start by writing down every part number of the ICs and pull the data sheets.  Then I start to trace out the design.   Most of the time, I have some idea where to look based on the symptoms and so I am only looking at small sections of the design. 

Just because schematics are not available for free off the internet doesn't mean they are not available.  I have written companies before and have been able to obtain schematics and service manuals. 

Can any of you also bring the 869 to show 0 Volts with a dc offset? Though there with the dual display less of an issue.

Does it really matter?  I saw it as more a gimmick than anything useful.   

[Neutrion]

This is a good throughout approach, but often one does not have the time for it, and have to start to measure around to get an idea. Of course it's not ideal.

Can any of you also bring the 869 to show 0 Volts with a dc offset? Though there with the dual display less of an issue.

Does it really matter?  I saw it as more a gimmick than anything useful.   

No, but I am interested. And the 869 is the competitor to the 789 within Brymen. (And actually not much of a competition in this price/quality range anywhere else...)

But generally it matters with the 789 because first I thought, that below a given displayed mV AC level I could feel myself "safe" but it seems that it is not the case.

I don't know how fast it could work in FW but if the meter would only switch internally to DC measurement, it should only check if its above 1 V DC in any any direction, and than it could display OL.

We will see.

[joeqsmith]

...
But generally it matters with the 789 because first I thought, that below a given displayed mV AC level I could feel myself "safe" but it seems that it is not the case.
...

Feel Safe?   It's rare I work on anything where I'm concerned with safety.  In those cases, I never feel safe!    I'm also not playing with mV. 

[joeqsmith]

Someone wrote me once about repairing toasters or such and they were all concerned with arc flash.    

[Neutrion]

Yes, that is why I used quote marks. So wanted to get a workaround for the missing overrange indication. But it seems that above 1V DC offset the displayed walue can be from 0 to anything, so there is no way of knowing it from the displayed value.

What do you think about the FW checking 1-2X a second the DC value? Would that be possible at all?
How much time does it take for the controller to switch from one mode to another? If I press the button it takes quiet a long time, so if this is HW related than that would be a no-go.

[Fungus]

Someone wrote me once about repairing toasters or such and they were all concerned with arc flash.   

Understandable. Anybody who's ever stuck a knife in a hot toaster to get the bread out knows what can happen in there.

[Neutrion]

The reason for getting the bread out only with proper CAT rated equipment!

[AVGresponding]

Someone wrote me once about repairing toasters or such and they were all concerned with arc flash.   

Understandable. Anybody who's ever stuck a knife in a hot toaster to get the bread out knows what can happen in there.

This is a thing? I mean to say, there are people out there who are foolish enough to do such a thing?

[Monkeh]

Someone wrote me once about repairing toasters or such and they were all concerned with arc flash.   

Understandable. Anybody who's ever stuck a knife in a hot toaster to get the bread out knows what can happen in there.

This is a thing? I mean to say, there are people out there who are foolish enough to do such a thing?

Have you met the average American?

[AVGresponding]

Someone wrote me once about repairing toasters or such and they were all concerned with arc flash.   

Understandable. Anybody who's ever stuck a knife in a hot toaster to get the bread out knows what can happen in there.

This is a thing? I mean to say, there are people out there who are foolish enough to do such a thing?

Have you met the average American?

I don't know. If I have, they didn't identify themselves as such.   

[Fungus]

Have you met the average American?

I don't know. If I have, they didn't identify themselves as such.   

(Insert George Carlin quote here...)

[bdunham7]

This is a thing? I mean to say, there are people out there who are foolish enough to do such a thing?

Think of it as a skill building exercise like that kid's game "Operation".

[bdunham7]

Have you met the average American?

I suppose you Brits do everything prim and proper like unplugging  your toasters before probing and shutting your car engines off before changing the fan belt?

https://youtu.be/BQhfcdQf1QA

[AVGresponding]

Have you met the average American?

I suppose you Brits do everything prim and proper like unplugging  your toasters before probing and shutting your car engines off before changing the fan belt?

https://youtu.be/BQhfcdQf1QA

I've done lots of dangerous things in my time. Some were for pay, some for duty, some for fun, and some out of ignorance. I can honestly say I've never done anything dangerous for toast.

[Per Hansson]

Someone wrote me once about repairing toasters or such and they were all concerned with arc flash.   

Understandable. Anybody who's ever stuck a knife in a hot toaster to get the bread out knows what can happen in there.

This is a thing? I mean to say, there are people out there who are foolish enough to do such a thing?

I'm willing to bet there are more people killed by toasters than sharks every year!

[AVGresponding]

Someone wrote me once about repairing toasters or such and they were all concerned with arc flash.   

Understandable. Anybody who's ever stuck a knife in a hot toaster to get the bread out knows what can happen in there.

This is a thing? I mean to say, there are people out there who are foolish enough to do such a thing?

I'm willing to bet there are more people killed by toasters than sharks every year!

I suppose we need something to scrape the flaky bits off the bottom of the gene pool.   

[joeqsmith]

I think it was an American who asked.

Not too surprised by by people getting a poke from their appliances.  I was surprised they were concerned about an arc flash.   I wonder if there's a special group that consider ANY spark an arc flash event.  Maybe the same group that always discusses safety here?  That would explain a lot.   

That can't be it because it seems many are not from the United States.   

[AVGresponding]

I think it was an American who asked.

Not too surprised by by people getting a poke from their appliances.  I was surprised they were concerned about an arc flash.   I wonder if there's a special group that consider ANY spark an arc flash event.  Maybe the same group that always discusses safety here?  That would explain a lot.   

That can't be it because it seems many are not from the United States.

"arc fault protection" is the latest buzz-phrase in electrical installation here. Lots of profit to be made implementing the new regs, which is a good thing, right?   

[2N3055]

I think it was an American who asked.

Not too surprised by by people getting a poke from their appliances.  I was surprised they were concerned about an arc flash.   I wonder if there's a special group that consider ANY spark an arc flash event.  Maybe the same group that always discusses safety here?  That would explain a lot.   

That can't be it because it seems many are not from the United States.

"arc fault protection" is the latest buzz-phrase in electrical installation here. Lots of profit to be made implementing the new regs, which is a good thing, right?   

There is a lot of confusion because there is a failure mode where in residential installations, sometimes bad connections cause local sparking and contact resistance overheating of connections, that can damage insulation to the point of it itself becoming conductive and starts heating by itself and can create fires.. They realized that this bad contact sparking could be detected by looking into current spectrum etc...

Some of that detection technology came from arch flash fault protection tech (current signature analysis), so marketing jumped at opportunity and used all the drama and buzzwords to make appear that is some sort of "distant cousin" of arc flash accidents.. And you should be afraid, very afraid.. Jerks... They call it "arc protection", not "arc flash fault protection" so technically they didn't say it is the same..

Slow smoldering of insulation and arc flash accidents shouldn't be confused...

Arc flash accidents cannot happen if there is not enough energy.
Non professional shouldn't work on anything where arc flash accident can happen.
And probably won't have access to anything where it can happen.

OTOH, people nowadays have large machinery in their garages, and it is tempting to try to repair failed VFD or welding inverter by yourself. Some of these can have serious DC capacitor bank, that can hurt you. Those can seriously hurt your face, hands, eyes and hearing...
When shorted, compared to real arc flash accident, they are mere large firecrackers compared to the block of C4 .
But you can get seriously hurt by firecracker if you put it next to your face or hold it in a closed hand.


 

[bdunham7]

"arc fault protection" is the latest buzz-phrase in electrical installation here. Lots of profit to be made implementing the new regs, which is a good thing, right?   

I just had a fire that did thousands of dollars of damage that was due to a defective device arcing internally.  I have AFCI breakers on certain circuits, but this was a 240VAC circuit for the swimming pool, so GFCI not AFCI is what is installed.  AFCI might just have prevented that fire. 

[joeqsmith]

There is a lot of confusion because there is a failure mode where in residential installations, sometimes bad connections cause local sparking and contact resistance overheating of connections, that can damage insulation to the point of it itself becoming conductive and starts heating by itself and can create fires.. They realized that this bad contact sparking could be detected by looking into current spectrum etc...

Some of that detection technology came from arch flash fault protection tech (current signature analysis), so marketing jumped at opportunity and used all the drama and buzzwords to make appear that is some sort of "distant cousin" of arc flash accidents.. And you should be afraid, very afraid.. Jerks... They call it "arc protection", not "arc flash fault protection" so technically they didn't say it is the same..

Slow smoldering of insulation and arc flash accidents shouldn't be confused...
...

I watch very few electronics channels and even less having to do with electrical circuits.  That said, I subscribed to John Ward's channel years ago and have watched several of his videos.   

He produced several on this subject.  I left him a comment after seeing one of his early ones and was glad to see him continue to provide more details about them.    I tried to procure that exact same part from Eaton but wasn't able to locate one in the USA. 

[Neutrion]

One more question about the BM789 which annoys me a bit:
The treshold for continuity is between 100Ohm and 420Ohm.
First I am not sure what this intervall means. But more importantly, isn't 420Ohm too high for finding shorts? I got used to 10-20Ohm but this one beeps all the time. Now I see that the 869 has also 200Ohm treshold but they doubled that with the 789. So there must be some application for it... What is it? I can't think it is just accidentally so high.
(The continuity voltage is 2,3 Volts measured with a multimeter.)

[bdunham7]

The treshold for continuity is between 100Ohm and 420Ohm.
What is it? I can't think it is just accidentally so high.

It might be that they use a low test current.  Unfortunately, continuity threshold is a spec that we will all never agree on, so the only solutions are to either live with what you get or find a meter that has selectable thresholds--which is pretty rare.

[2N3055]

"arc fault protection" is the latest buzz-phrase in electrical installation here. Lots of profit to be made implementing the new regs, which is a good thing, right?   

I just had a fire that did thousands of dollars of damage that was due to a defective device arcing internally.  I have AFCI breakers on certain circuits, but this was a 240VAC circuit for the swimming pool, so GFCI not AFCI is what is installed.  AFCI might just have prevented that fire.

House fires are serious enough. I wasn't saying that.
Those AFCI devices are definitely something that might save many houses and even lives..

It's just, that, large arc flash accident is a very violent explosive event that is similar to someone dropping bomb in how dangerous it is..

[joeqsmith]

If the laws mandate these devices and you end up with an electrical fire, are they liable?  Seems like they could be.

[bdunham7]

If the laws mandate these devices and you end up with an electrical fire, are they liable?  Seems like they could be.

They could be if the plaintiff could prove that the device did not perform as expected or required by the relevant standard, but that would be a lot more difficult than a strict liability case.  My guess is that AFCI manufacturers will face liability in house fire cases if they haven't already.  I'm not aware of any current such cases, but since most cases settle before they see the light of day, there could be many.

[Fungus]

My guess is that AFCI manufacturers will face liability in house fire cases if they haven't already.  I'm not aware of any current such cases, but since most cases settle before they see the light of day, there could be many.

There's a ton of fake devices out there:



How do you make 100% sure you aren't installing one?

[AVGresponding]

My guess is that AFCI manufacturers will face liability in house fire cases if they haven't already.  I'm not aware of any current such cases, but since most cases settle before they see the light of day, there could be many.

There's a ton of fake devices out there:



How do you make 100% sure you aren't installing one?

By having the work done by a reputable electrician using quality brand named goods from a reliable bricks and mortar source.

That's not a 100% guarantee by any means, but should lower the risk to very, very low levels. I haven't (yet) seen any counterfeit brand named items (Eaton, Schneider, Hager etc) from the big wholesalers (Edmundsons, CEF etc).

I have come across some fake BS1362 fuses, but unfortunately couldn't say where they came from.

[Neutrion]

The treshold for continuity is between 100Ohm and 420Ohm.
What is it? I can't think it is just accidentally so high.

It might be that they use a low test current.  Unfortunately, continuity threshold is a spec that we will all never agree on, so the only solutions are to either live with what you get or find a meter that has selectable thresholds--which is pretty rare.

OK, but what is the case for high Ohm values?  I found many discussions where it was pointed out that some meters have a too high test voltage (like the Fluke 87V with 7 V.) which sometimes can damage things but not much discussion about where you could use such a high Ohm value for continuity?
I want to get now a(....searching for the term... ) "Choice-supportive bias or post-purchase rationalization" supportive oppinion.

I only could think about getting through dirty  contacts, or something like this. Now the meter is fast at continuity testing, so if I constantly read the display I can have an Idea that it is not a short, but than I could do it without the buzzer function as well.

[Fungus]

I found many discussions where it was pointed out that some meters have a too high test voltage (like the Fluke 87V with 7 V.)

All those discussions are wrong.

It's not a test voltage, it's a test current, use Ohms law to figure out the voltage.

In the case of the Fluke the maximum voltage is 7V. If Ohms law gives you a number higher than that in the previous step then it will be limited to 7V and the test current won't be reached

[Neutrion]

So these are the discussions I found on this topic to not to start it again:
https://www.eevblog.com/forum/testgear/multimeter-with-low-continuity-test-voltage/
https://www.eevblog.com/forum/eda/pcb-reverse-engineering/msg668285/#msg668285
https://www.eevblog.com/forum/testgear/what-do-you-think-of-the-new-fluke-87-v-max/275/

I know this is not an argument, but I am happier with a lower voltage.  But still no good use for the high Ohm range!?      

[vqtrong]

Hi, I just bought a new BM789.

All functions are good. It is better then my Fluke 116 in measuring ripple voltage, as BM789 has Peak Hold (Crest).

Just one thing I concern is it comes with version 06, which is not the latest version (08 atm).

Is there any big update in 08 version?
Is there any way to update to the latest firmware?

And there is an icon (((D))) showing up in LCD if I follow this sequence:
In AC mode --> Enable VFD --> Change to Hz mode --> Enable REC (Min, Max)

[Neutrion]

That's strange. Can you go back to normal mode without any issues? Do the values change just like if the meter would work normaly? Or what does it do?
I don't want to screw my meter by accidently entering into some calibration mode, though I have the FW version 08.
"D" could stand either for "Dave", or maybe "detonator"?  Does it counting back when you activate it?
Just checking whether you can log into the forum a second time...

It's also interesting that it also has the small lightning symbol like some other meter, but it never shows it. Some legal disputes with Fluke?

[Kean]

I don't know if the fixes for FW 08 have been posted anywhere, but earlier ones are listed in a post by Dave at https://www.eevblog.com/forum/testgear/new-eevblog-bm786-multimeter/msg3624885/#msg3624885

From what Dave has said, firmware updates can only be done by factory or possibly by the distributor.

Interesting icon you found.  Not seen that in any of the docs.  It could be a bug, or a hidden feature not intended for end users.  D for diagnostics?  D for data?

[vqtrong]

That's strange. Can you go back to normal mode without any issues? Do the values change just like if the meter would work normaly? Or what does it do?
I don't want to screw my meter by accidently entering into some calibration mode, though I have the FW version 08.
"D" could stand either for "Dave", or maybe "detonator"?  Does it counting back when you activate it?
Just checking whether you can log into the forum a second time...

It's also interesting that it also has the small lightning symbol like some other meter, but it never shows it. Some legal disputes with Fluke?

Hi, I'm here ^^ That icon only disappears when turning the dial. Luckily there is no tick-tock sound or any counting-down timer

I don't know if the fixes for FW 08 have been posted anywhere, but earlier ones are listed in a post by Dave at https://www.eevblog.com/forum/testgear/new-eevblog-bm786-multimeter/msg3624885/#msg3624885

From what Dave has said, firmware updates can only be done by factory or possibly by the distributor.

Interesting icon you found.  Not seen that in any of the docs.  It could be a bug, or a hidden feature not intended for end users.  D for diagnostics?  D for data?

I think it is for Data. I have not seen any Brymen DMM shows that icon when connected to PC. Is there any one having BM896s seeing this icon?

[vqtrong]

Searching around, I found a post from Cymaphore
https://www.eevblog.com/forum/testgear/new-eevblog-bm786-multimeter/msg3625880/#msg3625880
in which he said "Data port for BT is present under the display but unused in the standard BM789."

Have seen anyone hacked to that data port

[Fungus]

From what Dave has said, firmware updates can only be done by factory or possibly by the distributor.

Yep. Calling it "firmware" is a bit of a misnomer. It's more of a ROM.

Brymen sent joe a firmware "update" in the form of a new chip to be soldered in. The calibration was a bit off after he fitted it.

TLDR: Whatever "firmware" you got with the meter is what you get.

[Fungus]

Interesting icon you found.  Not seen that in any of the docs.

It looks like a some sort of wireless data transfer icon to me.

Does anybody with newer firmware get the icon with that button sequence?

[Kean]

Yep. Calling it "firmware" is a bit of a misnomer. It's more of a ROM.

Brymen sent joe a firmware "update" in the form of a new chip to be soldered in. The calibration was a bit off after he fitted it.

TLDR: Whatever "firmware" you got with the meter is what you get.

Nope, it is not ROM.
It can be updated, but only with the right equipment as provided by Brymen - so not by the end user. (at least not yet)

[Neutrion]

That's strange. Can you go back to normal mode without any issues? Do the values change just like if the meter would work normaly? Or what does it do?
I don't want to screw my meter by accidently entering into some calibration mode, though I have the FW version 08.
"D" could stand either for "Dave", or maybe "detonator"?  Does it counting back when you activate it?
Just checking whether you can log into the forum a second time...

It's also interesting that it also has the small lightning symbol like some other meter, but it never shows it. Some legal disputes with Fluke?

Hi, I'm here ^^ That icon only disappears when turning the dial. Luckily there is no tick-tock sound or any counting-down timer

I am glad that you survived. Brymens are generally safe meters I am sure it would tell us to take cover!

But I still did not dare to try it. What does the display show when the "d" appears? Can you normally measure voltage with it? All the other options are working?

Does anyone know what the processor(not the frontend) of the meter is? If we suppose its one of the Hycons, what could be the price difference to get something with decent memory? (Apart from the current crisis of course) The are plenty of MCU on the market I suppose.
Also interesting why the 789 needs the extra shielding and pots compared to the 786.

And one general question about multimeter backlights: The reason no pwm is used to not to screw the measurements? Because if all the manufacturer are worrying about the power consumption of the backlight, why not implementing a pwm some sort?

[AlexTee]

May I ask owners of this lovely DMM to test something for me before I order one for myself?
According to the user manual the continuity test should turn on audible sound if the circuit resistance is lower than 100 Om with response time of 100 μs.
Could you please confirm this statement? Probe a resistor of say 10-20 Om, and see if the buzzer comes on immediately. Because as I figured out recently, not all meters can do that, some turn on buzzer with significant delay if there's a small resistance present.

[Caliaxy]

According to the user manual the continuity test should turn on audible sound if the circuit resistance is lower than 100 Om with response time of 100 μs.
Could you please confirm this statement? Probe a resistor of say 10-20 Om, and see if the buzzer comes on immediately.

Yes, it beeps with no delay up to at least 120 Ohms.

[AlexTee]

Nice. Now, the question that still remains somewhat unclear to me is what's the idea behind setting such a high threshold of up to 420 ohm for continuity test. It means I need to throw a glance at the screen every single time I hear the beep, because it might happen to be just a resistor of 200 ohms along the tested path.

I re-read last pages, and found user Fungus explained that this is related to using a very small current in this mode. So, if I understand it right, the small amount of current is the root cause why the threshold is pushed high enough and enclosed in a range for the meter to detect sufficient voltage deviation over the tested path to distinguish between short/open condition. Sounds legit?

[HAO]

Hi everyone. I have BM869s and BM789. Both meters have not good battery life.

BM869s :
Power Supply: Single Alkaline 9V battery
Power Consumption: 6.5mA typical; 8mA for VFD ranges
Low Battery: Low Battery: Below approx. 7V

BM789
Power Supply: 1.5V AAA Alkaline battery x 3
Power Consumption: 10mA typical for AC & AC+DC Voltage/Current functions; 8mA
typical for other functions
Low Battery: Below approx. 3.7V

Do they work well with NiMH cells and not show low battery indicator?
AAA Eneloop for BM789
Energizer NH22-175 or Ansmann Akku 9V 300 mah for BM869s.

I also have Fluke 87V and Fluke 117. Should I stay with Energizer, Panasonic Alkaline or switch to rechargeable batteries ?

[joeqsmith]

Hi everyone. I have BM869s and BM789. Both meters have not good battery life.

So far, the best meter I have looked at for battery life was the free meters we get from HF and the Fluke 17B+.  I estimate both around 1200 hours.   Much better than the two meters you mention.  The worse I have seen was the UNI-T UT181A.  Maybe 40 hours between charges on a new battery, and the meter can't be used when it is charging which is done overnight.   

I have been using rechargeable Lithium-ion batteries with the BM869s.   The ones I have are marked 600mAh.  I've not had any problems using them. 

[EEVblog]

Searching around, I found a post from Cymaphore
https://www.eevblog.com/forum/testgear/new-eevblog-bm786-multimeter/msg3625880/#msg3625880
in which he said "Data port for BT is present under the display but unused in the standard BM789."

Have seen anyone hacked to that data port

Not possible because the firmware does not output and signal you can use.
I requested this way back in the development of the meter, saying it would be a popular user retro-fit, and Brymen agreed. But they couldn't get it done for some reason and the feature was not implemented.

[Antrus]

Greetings to all. I have a question about measuring resistance with this multimeter. At first it shows a slightly lower value, after a second the final one. Is it so for everyone? Firmware 78908.In reviews, he showed the value immediately.

[Grandchuck]

Sounds like capacitance is involved?

[Antrus]

Do you mean the capacitance is the internal capacitance of the multimeter? Since only the resistor is measured. This happens only in auto-range mode when measuring from several kilo-ohms.

[Grandchuck]

I meant capacitance in the circuit under test.  I have no other suggestions.

[supertrabuco]

Hello, the kings of the east have visited me and have left me an EEVBlog BM786.
Greetings and happy new year to all the electronic family ;

[Caliaxy]

Greetings to all. I have a question about measuring resistance with this multimeter. At first it shows a slightly lower value, after a second the final one. Is it so for everyone? Firmware 78908.In reviews, he showed the value immediately.

Mine does pretty much the same thing - it shows an intermediary value (pretty close to the final one) after about a second and a final stable value after about another second (tried to measure a 2k2 resistor in auto-range). I wouldn't worry too much about this, just be patient and keep in mind that the first value shown is not the final one.

Many low cost meters do exactly the same (also tried Uni-T UT61e). BM235 doesn't, it shows directly a final value and so does Fluke 87V (which also measures about 25% faster than BM789).

BM789 is a very accurate (and great for its price) meter, but not exactly a joy to use. Still a nice one.

[Antrus]

It would be better if he did not show this intermediate value. I am sure that the firmware can fix this.

[Antrus]

Range + hold + power on   is this bm789 calibration mode?

[Caliaxy]

It would be better if he did not show this intermediate value.

Agree.

I am sure that the firmware can fix this.

Let's hope so, for the future buyers. It won't help you and me, because the firmware is not user updateable in BM789.

[Neutrion]

I can confirm this too, however, it's not an issue, as I would normally wait anyway more than a second to let the values stabilize. Especially if I need an exact reading and not just an approximation. (Which is displayed really fast.) On manual range the problem disappears.

Anyone having a newer FW than 08 so far?

BM789 is a very accurate (and great for its price) meter, but not exactly a joy to use. Still a nice one.

I disagree, for me it's a joy to use.   The super big display numbers, and the nice warm white background light  are hard to beat for any other meters in the same display count and meter size category. (Regarding the usage feelings.)
And it's magnetic holder is also unmatched, if we are talking about field use.
Poor little meter, its one of the best on the market, but in this topic it almost only got beating for some very minor issues.

[Monkeh]

Poor little meter, its one of the best on the market, but in this topic it almost only got beating for some very minor issues.

Any excuse for the Fluke fanboys.

[AVGresponding]

Poor little meter, its one of the best on the market, but in this topic it almost only got beating for some very minor issues.

Any excuse for the Fluke fanboys.

I'm a Fluke fanboy, but I have no bad words to say about the BM789, or any other Brymen meter I've seen. Hope that doesn't mean I have to hand my membership back in...

[Fungus]

I'm a Fluke fanboy, but I have no bad words to say about the BM789, or any other Brymen meter I've seen. Hope that doesn't mean I have to hand my membership back in...

I'm afraid it does.

[Neutrion]

It was actually me who broght up some "problems" and I am not a Fluke fanboy, but when someone reads through the topic could get the feeling that the meter is full of problems while it's actually not.
Just the Brymen fanboys Fluke-fanboy-like love is missing, about warm feelings, love, and happiness while we are mearusing with the meter, and other peoples feeling when we show them what we have.

But I can begin: When I am looking at nice crisp display of the BM789, it gives me confidence, not just in the measurement, in life in general. And such a meter is hard to design!

[Fungus]

It was actually me who broght up some "problems" and I am not a Fluke fanboy, but when someone reads through the topic could get the feeling that the meter is full of problems while it's actually not.
Just the Brymen fanboys Fluke-fanboy-like love is missing, about warm feelings, love, and happiness while we are mearusing with the meter, and other peoples feeling when we show them what we have.

Yep. Brymen fanboys aren't afraid to dig deep and find the tiny imperfections (if any).

Fluke fanboys are content to just put the meter on a pedestal and worship.

[Neutrion]

I'm afraid I have to agree.

And to not to start division, and endless debates, I hope we can all agree on the fact, that the Brymen fanboys and their meters (only the Brymen ones!) are generally better in EVERYTHING!

(I just became a Brymen fanboy, the official papers just arrived.)

On the other hand, there are still fields where we could learn something, I started to build a pedestral.  Just finally have to get the courage to peal off the foil from the display of the BM789.

[Fungus]

Just finally have to get the courage to peal off the foil from the display of the BM789.

The first thing I do with all my meters is cut a piece of smartphone screen protector to size and stick it on the display.

There's a few meters with a screens which are curved in both X and Y and the protector won't stay on, I'm looking at you, Fluke 101.

I hope the curvy modern Brymens aren't like that.

[Neutrion]

I was also thinking about that, but it is a bit curvy. Not sure either how it will affect the display quality.

[Neutrion]

So, it boggles my mind that none of the forum members from this topic pointed us sofar to this discussion:
https://www.eevblog.com/forum/testgear/brymen-multimeters-fault/       

Where Joes Smith and some others measured the mVAC and AC+DC scale of many handheld meters, and the result was, (regarding the missing OL indication) that most of the meters(including most, if not all the Flukes as well) do this on the mV scale, and there was practically only one meter which seemed to overrange reliably instead showing false values and it was the Extech EX540.


So some of you mentioned that this is a problem with a few meters sometimes, but it seems everybody forgot about the topic where the exact measurements were mentioned years ago.   

It makes me feel  much better about the  BM789, because it seems that it is possibly not so trivial at all to solve this issue, and many other much more expensive meters are also plagued.   

Still it would be nice to know wether there is any new FW, and test this stuff with it.

[vqtrong]

...
Still it would be nice to know wether there is any new FW, and test this stuff with it.

I really really want to know if there is any way to update new FW, even in a hacky way. 'cause I'll cry so much when someone has a new FW and plays with it   

[joeqsmith]

So, it boggles my mind that none of the forum members from this topic pointed us sofar to this discussion:
https://www.eevblog.com/forum/testgear/brymen-multimeters-fault/       
..

   Why? The search engine is available for everyone to use.   It boggles my mind that anyone would expect other members to do their research. .... well, not so much anymore..    Wait a year, it will come up again.   This time I will expect you to point out all of the previous threads that cover it. 

***
May as well include the more recent links as well. 
https://www.eevblog.com/forum/testgear/brymen-869s-dc-offset-issue-when-reading-acmv/
https://www.eevblog.com/forum/testgear/brymen-bm789/msg3696892/#msg3696892

[Antrus]

Judging by the review on YouTube, bm785 measures resistance faster than bm789. A little embarrassing. They are essentially the same.

[Neutrion]

So, it boggles my mind that none of the forum members from this topic pointed us sofar to this discussion:
https://www.eevblog.com/forum/testgear/brymen-multimeters-fault/       
..

   Why? The search engine is available for everyone to use.   It boggles my mind that anyone would expect other members to do their research. .... well, not so much anymore..    Wait a year, it will come up again.   This time I will expect you to point out all of the previous threads that cover it. 

***
May as well include the more recent links as well. 
https://www.eevblog.com/forum/testgear/brymen-869s-dc-offset-issue-when-reading-acmv/
https://www.eevblog.com/forum/testgear/brymen-bm789/msg3696892/#msg3696892

I actually read a lot about Brymen meters before, but somehow that topic was not found.
But yes normally in such cases if I remember that the whole issue was discussed before in detail I remind
the others of that.

Judging by the review on YouTube, bm785 measures resistance faster than bm789. A little embarrassing. They are essentially the same.

785, or 786? I suppose 786, but  the 785 should exist as well.



Little update: I put a relative thin screen protector foil on the screen, and it works well, and does not affect the screen negatively. But it really has to be thin, as the screen is slightly curved.

[Antrus]

Yes review in Russian. In general, somewhere should be trial models 783,784,787,788 and BT. Probably decided not to release them.

[Antrus]

Calibration is entered by pressing the button rec + ohm .

[Neutrion]

Sorry for the late reply!
Do you know aything more of the calibration procedure? I watched the russian Video, but it is hard to tell whether there are any speed differences.

[EEVblog]

Yes review in Russian. In general, somewhere should be trial models 783,784,787,788 and BT. Probably decided not to release them.

The bluetooth version was supposed to be released at launch time but they had issues implementing it. I haven't heard anything about it for a long time.

[Antrus]

Does anyone know what changes are in firmware 78909 and 78910?

[Neutrion]

Any info from anyone regarding these two new fw version? (09, 10) On Brymens site, it is not even offered for download.

[J-R]

I'm not aware of any Brymen DMMs that have updateable firmware...

[Neutrion]

The new ones (78x) have updateable FW, Dave even made a video about it, but it is not aviable for us comon people, only distributors.
Although a changelog would be nice. Whether the issues we touched on in this topic were resolved or not.
I couldn't find any FW change info in the 786 topic either.

[NoMoreMagicSmoke]

Is the Hycontek firmware readable from the chip?

[EEVblog]

I'm not aware of any Brymen DMMs that have updateable firmware...

The 78x series does.

[NoMoreMagicSmoke]

I'm not aware of any Brymen DMMs that have updateable firmware...

The 78x series does.

Dave are you allowed to say whether that programmer can/cannot read from the chip?

[EEVblog]

Dave are you allowed to say whether that programmer can/cannot read from the chip?

IIRC I tried that when I played with it and got nothing. So I presume it's protected.

[NoMoreMagicSmoke]

IIRC I tried that when I played with it and got nothing. So I presume it's protected.

I can't say I am surprised. With the lengths they went to obscure which chips are used I would be a bit perturbed if there wasn't some type of read protection. It's a bit of a bummer that you can't at least read out the EEPROM values within the CPU to enable repairability. Of course there are safety issues with doing modifications/repairs on a DMM, so probably for the best in this case.

[J-R]

I'm not aware of any Brymen DMMs that have updateable firmware...

The 78x series does.

OK, I have a BM789 that has old firmware, how can I get it updated?

[Neutrion]

Dave might does it for free.   Anyway I would also like to know, but I am afraid if there is no serious safety issue with the FW there is no such a service from Brymen.

Dave: Do you have any info about these new firmwares? Or any new ones for the 786?

[EEVblog]

OK, I have a BM789 that has old firmware, how can I get it updated?

As I mentioned in the video, you can't. It's just too expensive to ship it back and forth for a firmware update. I don't think any Brymen dealer is going to offer this as a service.
This is what I was afraid of with releasing that video, people asking to have their firmware updated.
You have to consider the BM78x like all other Brymen's, the firmware you get when you buy it is the firmware you have.

[EEVblog]

Dave: Do you have any info about these new firmwares? Or any new ones for the 786?

I have asked but I have not heard anything yet.

[J-R]

OK, I have a BM789 that has old firmware, how can I get it updated?

As I mentioned in the video, you can't. It's just too expensive to ship it back and forth for a firmware update. I don't think any Brymen dealer is going to offer this as a service.
This is what I was afraid of with releasing that video, people asking to have their firmware updated.
You have to consider the BM78x like all other Brymen's, the firmware you get when you buy it is the firmware you have.

Yes, I watched your video when it came out, as well as Joe's where he swapped chips.

So technically every Brymen probably can have it's firmware updated, but effectively none of them can from a user perspective.

But of course the BM78x sounds like it could be if someone takes a whack at it.

[EEVblog]

So technically every Brymen probably can have it's firmware updated, but effectively none of them can from a user perspective.

No, all other Brymen models use an OTP processor.
The 78x series is a new design that includes a flash based processor.

[EEVblog]

FYI

78608 version is only to enlarge allowance range for OHM hardware offset to reduce production failure rate. It does not affect meter performance.

[Fungus]

Somebody just opened a can of worms... 

[Antrus]

Found another annoying thing. In the video on YouTube, and checked it myself. In the region of 650 ohms and 6500 ohms, the multimeter cannot measure resistance in automatic mode. In firmware 78908-78910.

[joeqsmith]

Found another annoying thing. In the video on YouTube, and checked it myself. In the region of 650 ohms and 6500 ohms, the multimeter cannot measure resistance in automatic mode. In firmware 78908-78910.

I was able to replicated this with 08 and have contacted Brymen.   

[NoMoreMagicSmoke]

What's the symptom?

[Neutrion]

Did you manage to update the FW, or you have different meters?
BTW did the dc offset sinewave overload indication on the mV scale issue changed with any of the FWs? There was a debate whether it would be possible at all to solve that with any FW update.

[Monkeh]

Did you manage to update the FW, or you have different meters?

Only dealers have the programming tool and access to the firmware images, so it seems rather unlikely he updated it.

[joeqsmith]

Assuming you are addressing me,  I talked about the firmware updates throughout the videos for both the 786/9 meters.   As for DC biased waveforms, again, I supplied all that detail in the videos.

There was a bit of a discussion here (more than once) where a few beginners were asking about DC biased waveforms and I dove into it a bit further for them.  That's all online here. 

[Neutrion]

Sorry, I was actually adressing Antrus, but something went wrong it seems. But as long I remember you also have the 08.
Antrus seems to have tried two never versions.

[Antrus]

I have firmware 8. The person on YouTube has firmware 10. Of course, I haven't updated anything. The multimeter is almost 2 years old, but the manufacturer has not yet noticed such an obvious error in the firmware. Perhaps this resistance range is not needed by anyone.

[Constantin]

I have  Brymen BM 789, firmware 09 and It has no problems with the 650 Ohm and 6.5 kOhm measurement.

[Antrus]

Try measuring the variable resistor. In the range of 650-660 ohms in automatic mode, he goes crazy.

[joeqsmith]

I have firmware 8. The person on YouTube has firmware 10. Of course, I haven't updated anything. The multimeter is almost 2 years old, but the manufacturer has not yet noticed such an obvious error in the firmware. Perhaps this resistance range is not needed by anyone.

Did you bring it to their attention, or assume that posting to the EEVBLOG forum was the correct way to handle it? 
***
If you did write them, what was their response?  They have been very responsive when I have ask them questions in the past.

[Constantin]

Yes, I was able to find magical resistance, about 653 Ohms in my case :-)

[NoMoreMagicSmoke]

Try measuring the variable resistor. In the range of 650-660 ohms in automatic mode, he goes crazy.

I wonder if this is an issue with just some meters? I just tried with a potentiometer and s sweep from 600-700 6.0k - 7.0k could not cause the meter to freak out. My meter is 08 and it ranges up at 66000 counts and down at 61000 counts.

I did only have a 1 turn pot available though so it's possible it's a very narrow range and I didn't have the resolution on the potentiometer to hit the sweet spot.

[EEVblog]

Try measuring the variable resistor. In the range of 650-660 ohms in automatic mode, he goes crazy.

I wonder if this is an issue with just some meters? I just tried with a potentiometer and s sweep from 600-700 6.0k - 7.0k could not cause the meter to freak out. My meter is 08 and it ranges up at 66000 counts and down at 61000 counts.

I did only have a 1 turn pot available though so it's possible it's a very narrow range and I didn't have the resolution on the potentiometer to hit the sweet spot.

No issue at all on a new BM786 with 78608 firmware, or 78607 using my decade resistance box. It's just rock solid at 0.65x display.

[joeqsmith]

Try measuring the variable resistor. In the range of 650-660 ohms in automatic mode, he goes crazy.

I wonder if this is an issue with just some meters? I just tried with a potentiometer and s sweep from 600-700 6.0k - 7.0k could not cause the meter to freak out. My meter is 08 and it ranges up at 66000 counts and down at 61000 counts.

I did only have a 1 turn pot available though so it's possible it's a very narrow range and I didn't have the resolution on the potentiometer to hit the sweet spot.

No issue at all on a new BM786 with 78608 firmware, or 78607 using my decade resistance box. It's just rock solid at 0.65x display.

That's odd.   I had to hunt for it but was able to find a spot were the 789 will hunt indefinitely as suggested by the OP.   

[joeqsmith]

I should add that the 789 shown was the last meter I tested which had been setup at the factory.  It is not one of the meters I had swapped parts on.

Also, maybe a bit more to the story.  Looking at one of the 786 meters, w/ 78605 which had modified with the latest parts of that time, does not exhibit this problem. 

As I mentioned, I had forwarded this information to Brymen.   

[GAVER]

I confirm the problem.78909

[EEVblog]

I should add that the 789 shown was the last meter I tested which had been setup at the factory.  It is not one of the meters I had swapped parts on.
Also, maybe a bit more to the story.  Looking at one of the 786 meters, w/ 78605 which had modified with the latest parts of that time, does not exhibit this problem. 
As I mentioned, I had forwarded this information to Brymen.

The 786 and 789 firmware bases are different. Possible it exists in one and not the other.
I've gone up by 1 ohm increments from 600ohm to 680ohm and there is no problem.

[PushUp]

Unfortunately, the "new" Benning MM 7-2 with "FW 72001" has the same problem/bug:







I found the spot with my "Vishay Precision Potentiometer" immediately @ 661.x Ohm,....







....however, the MM 7-2 does not autorange for ever, but simply shows OL after autoranging (it is not as fast as the other DMMs).
When I pull the test leads out of the MM 7-2 and put them back in, it shows an interim value before going back straight to OL.







After that I only had to swap the test leads into the Fluke 289:






..., the Keysight U1273A...


foto fun kostenlos



...and the PM XTRA,...







...which have not any issue, showing the "right" resistance instantly depending on its precision.




...and back it goes...! 



Cheers! 

[joeqsmith]

I should add that the 789 shown was the last meter I tested which had been setup at the factory.  It is not one of the meters I had swapped parts on.
Also, maybe a bit more to the story.  Looking at one of the 786 meters, w/ 78605 which had modified with the latest parts of that time, does not exhibit this problem. 
As I mentioned, I had forwarded this information to Brymen.

The 786 and 789 firmware bases are different. Possible it exists in one and not the other.
I've gone up by 1 ohm increments from 600ohm to 680ohm and there is no problem.

Based on the posts, it seems so. 

I have confirmation that Brymen is now working on it.   Guessing the OP did not contact them as it did not seem like they were made aware of the problem until now.      I'll post an update when I have it.   

[TCbystander]

....however, the MM 7-2 does not autorange for ever, but simply shows OL after autoranging (it is not as fast as the other DMMs).
When I pull the test leads out of the MM 7-2 and put them back in, it shows an interim value before going back straight to OL.

Seems like you were using the continuity function, not the resistance function instead

[coromonadalix]

hmmmm seems like it  since you have the continuity beep  logo on the screeen ??

[PushUp]

I am quite sure, that I was in Ohms mode, when I did my decisive measurement, as I saw different values, when I searched for the 660 Ohms region and have just forgotten to pay attention to the settings, when doing the photosession behind the other DMMs, after playing around with the buttons of the MM 7-2, which also keeps the last settings...

As I have already sent the Benning back home, I cannot repeat my measurement. However, as I don't want to ruin Bennings sellings, I hope that I was mistaken and quote the assumption of innocence for Benning with its MM 7-2 and their FW 72001, that they have paid attention to the mentioned bug, as they should know, which older Brymen-FW they use as their own first FW-Version "72001"...

Next time I will make a video for myself, having a better proof of concept, as I am also annoyed about my lousy documentation, but as I will not buy a Brymen/Benning again, maybe someone else is willing to repeat the test, because what I can definitely say is, that I am totally happy with my aleady bought handheld DMMs and don't see any reason to buy another one... 

Sorry & Cheers! 

[Antrus]

And what will be the correction of this error on the part of the manufacturer? Previous buyers will stay with her. Maybe the controller can be read and flashed with a programmer from Stm32?
Why is there no BRYMEN representative on this forum?

[GAVER]

Дрючить техподдержку Brymen
, я уже дрючу!
Druchit  tech support Brymen
I'm already druchu!

[EEVblog]

And what will be the correction of this error on the part of the manufacturer? Previous buyers will stay with her. Maybe the controller can be read and flashed with a programmer from Stm32?
Why is there no BRYMEN representative on this forum?

Every single Brymen meter ever made before this 78x series (and many other brands including the biggies like Fluke) do not allow firmware updating, so you are stuck with the version you bought.
There is no manufacturer or dealer re-programming service offered.

[joeqsmith]

....however, the MM 7-2 does not autorange for ever, but simply shows OL after autoranging (it is not as fast as the other DMMs).
When I pull the test leads out of the MM 7-2 and put them back in, it shows an interim value before going back straight to OL.

As I have already sent the Benning back home, I cannot repeat my measurement.

Someone commented how they were using the LoZ to look at a battery, then measuring the battery with the standard voltage mode and would see the voltage slowly drift upward.  They were blaming the DMM and also returning it.   I think that's the jist anyway.  I am waiting for them to make a short video showing it. 

[AVGresponding]

....however, the MM 7-2 does not autorange for ever, but simply shows OL after autoranging (it is not as fast as the other DMMs).
When I pull the test leads out of the MM 7-2 and put them back in, it shows an interim value before going back straight to OL.

As I have already sent the Benning back home, I cannot repeat my measurement.

Someone commented how they were using the LoZ to look at a battery, then measuring the battery with the standard voltage mode and would see the voltage slowly drift upward.  They were blaming the DMM and also returning it.   I think that's the jist anyway.  I am waiting for them to make a short video showing it.

Really? I can understand the buyer being too dumb to understand what they are doing (if it's as you describe), but surely the seller should have some basic level of knowledge of the products they are supplying..? I would refuse a return on this basis anyway.

[joeqsmith]

Really? I can understand the buyer being too dumb to understand what they are doing (if it's as you describe), but surely the seller should have some basic level of knowledge of the products they are supplying..? I would refuse a return on this basis anyway.

Thank you for asking.  We are talking the WWW and there can certainly be differences in language, culture, education and our friendly trolls from time to time.  Lots of dynamics and you may interpret the discussion differently.   I welcome the input. 

[floobydust]

[...] Every single Brymen meter ever made before this 78x series (and many other brands including the biggies like Fluke) do not allow firmware updating, so you are stuck with the version you bought.
There is no manufacturer or dealer re-programming service offered.

I think Brymen needs to acknowledge the need to update the firmware in their multimeters. With feature sets now this complicated, updates are to be expected, the days of OTP are thankfully gone. There's not a lot of secret sauce in the firmware and supposedly they have custom silicon as well.
If Brymen is scared then let authorized dealers or a repair depot do it. Oh wait there's none in North America.

Fluke had top quality firmware, it was tested thoroughly, there is no comparison in that regard.

[AVGresponding]

Really? I can understand the buyer being too dumb to understand what they are doing (if it's as you describe), but surely the seller should have some basic level of knowledge of the products they are supplying..? I would refuse a return on this basis anyway.

Thank you for asking.  We are talking the WWW and there can certainly be differences in language, culture, education and our friendly trolls from time to time.  Lots of dynamics and you may interpret the discussion differently.   I welcome the input.

I see. Looks like you are doing your best to explain to someone with zero knowledge why battery voltage drops when loaded, and then recovers once the load is removed... I wish you luck!

[AVGresponding]

[...] Every single Brymen meter ever made before this 78x series (and many other brands including the biggies like Fluke) do not allow firmware updating, so you are stuck with the version you bought.
There is no manufacturer or dealer re-programming service offered.

I think Brymen needs to acknowledge the need to update the firmware in their multimeters. With feature sets now this complicated, updates are to be expected, the days of OTP are thankfully gone. There's not a lot of secret sauce in the firmware and supposedly they have custom silicon as well.
If Brymen is scared then let authorized dealers or a repair depot do it. Oh wait there's none in North America.

Fluke had top quality firmware, it was tested thoroughly, there is no comparison in that regard.

Maybe just wishful thinking, but I'd prefer a meter that works straight away, and not one that might work 7 firmware patches and 2 years down the line. Much of the time the issues Joe and others uncover are really not edge cases and should really be picked up at the R&D stage. This is not an issue exclusive to Brymen, either. The Fluke 87V has a firmware fault, that afaik exists even today, and has been explored on this forum. It's not a serious one, but it exists nevertheless.

I'd also worry that a used meter purchased off ebay or wherever, might have non-manufacturer firmware that may even cause safety issues, and I expect the manufacturers have similar concerns, as well as wanting to protect their IP of course.

[NoMoreMagicSmoke]

Fluke had top quality firmware, it was tested thoroughly, there is no comparison in that regard.

I am not sure this is really a fair statement. Fluke is not immune to rare firmware bugs and has had some themselves (87V GSM bug???). There have been a number of reviewers and this meter has been out for a couple of years now before this issue was found. I have a BM789 with firmware version 08 and I attempted to reproduce this and I could not with my meter. This means that either this issue does not affect all BM789 meters, or it is an extremely narrow scoped issue.

I guarantee that anything with software has edge case bugs that if you search for long enough can be uncovered. The fact that someone can find an obscure bug does not mean that the product is inferior. The company's response though does separate the good from the bad.

[joeqsmith]

....
The company's response though does separate the good from the bad.
...

[floobydust]

Fluke had top quality firmware, it was tested thoroughly, there is no comparison in that regard.

I am not sure this is really a fair statement. Fluke is not immune to rare firmware bugs and has had some themselves (87V GSM bug???). There have been a number of reviewers and this meter has been out for a couple of years now before this issue was found. I have a BM789 with firmware version 08 and I attempted to reproduce this and I could not with my meter. This means that either this issue does not affect all BM789 meters, or it is an extremely narrow scoped issue.

I guarantee that anything with software has edge case bugs that if you search for long enough can be uncovered. The fact that someone can find an obscure bug does not mean that the product is inferior. The company's response though does separate the good from the bad.

Fluke I thought it was a hardware bug a very long time ago, sensitivity to RF and the recall and ferrite bead added.

Basic F/W testing includes the boundary conditions. I'm not sure why a range change is even necessary here unless they have a quality problem with the Ohms current source variations and the DMM IC. You'd have to measure the Ohms test current when it's hunting and see it's likely moving around.
I'd be upset to spend ~$80 to ship to Taiwan for a F/W update. If a bug is found that includes a safety issue, Brymen would have to eat it.

[EEVblog]

If Brymen is scared then let authorized dealers or a repair depot do it. Oh wait there's none in North America.

And who's going to pay for all the shipping and time?

Fluke had top quality firmware, it was tested thoroughly, there is no comparison in that regard.

https://www.eevblog.com/forum/testgear/fluke-87v-87-5-87-v-gsm-interference-fluke-says-firmware-flash-fixes-it/

I used to have a 70 series Fluke at work that had an autoranging problem when meanruing mains transformer primaries, it just kept autoranging, displaying nothing.

[EEVblog]

I'd also worry that a used meter purchased off ebay or wherever, might have non-manufacturer firmware that may even cause safety issues, and I expect the manufacturers have similar concerns, as well as wanting to protect their IP of course.[/color][/size][/b]

Yes, these concerns have been cited to me by Brymen, Kane, Fluke, and Keysight.

[EEVblog]

I guarantee that anything with software has edge case bugs that if you search for long enough can be uncovered. The fact that someone can find an obscure bug does not mean that the product is inferior. The company's response though does separate the good from the bad.

And what should that response be?
Fluke for example very quietly fixed it in production units about 12 months later and made no mention of it at all to new or existing customers. A customer would have no idea what version they were buying.

[NoMoreMagicSmoke]

And what should that response be?
Fluke for example very quietly fixed it in production units about 12 months later and made no mention of it at all to new or existing customers. A customer would have no idea what version they were buying.

A good first step is to take interest in the issue to understand it's impact to the customer and the root cause. After that the response should be commensurate with the impact. Ex. If it's a safety issue then there should be a recall. If it's an edge case that has a material affect on a small number of users it should be handled as warranty on a case by case basis. If it's an oddity quirk that does not have a material effect on end users fix in new.

[Monkeh]

I used to have a 70 series Fluke at work that had an autoranging problem when meanruing mains transformer primaries, it just kept autoranging, displaying nothing.

I've seen a number of meters do similar, it's not uncommon. People just like to get worked up over things.

[floobydust]

If Brymen is scared then let authorized dealers or a repair depot do it. Oh wait there's none in North America.

And who's going to pay for all the shipping and time?

Fluke had top quality firmware, it was tested thoroughly, there is no comparison in that regard.

https://www.eevblog.com/forum/testgear/fluke-87v-87-5-87-v-gsm-interference-fluke-says-firmware-flash-fixes-it/

I used to have a 70 series Fluke at work that had an autoranging problem when meanruing mains transformer primaries, it just kept autoranging, displaying nothing.

Is your position that the firmware is carved in stone, users have to suck it up if there are (non-safety related) bugs, you are stuck with the version you bought?
I can't see Brymen making it to the next level with say a Bluetooth stack, that is carved in stone.

It's not really the bug that is a concern, it's the software testing coming across as weak. Autoranging is not rocket science. You flowchart and test at the boundary conditions such as a range change, and a range change gone wrong. And this issue is only a problem for people working in the 600 ohm world it seems.

A repair depot in North America would not burden customers with the crazy shipping costs/customs paper work to Taiwan. It seems to be that ____ exclusivity agreement blocking that. I think they're playing in dangerous waters, with no formal sales/distribution a recall would be a disaster.

I haven't ever been bitten by Fluke multimeter bugs, and see they dealt with them as a warranty repair.

[NoMoreMagicSmoke]

If Brymen is scared then let authorized dealers or a repair depot do it. Oh wait there's none in North America.

And who's going to pay for all the shipping and time?

Fluke had top quality firmware, it was tested thoroughly, there is no comparison in that regard.

https://www.eevblog.com/forum/testgear/fluke-87v-87-5-87-v-gsm-interference-fluke-says-firmware-flash-fixes-it/

I used to have a 70 series Fluke at work that had an autoranging problem when meanruing mains transformer primaries, it just kept autoranging, displaying nothing.

Is your position that the firmware is carved in stone, users have to suck it up if there are (non-safety related) bugs, you are stuck with the version you bought?
I can't see Brymen making it to the next level with say a Bluetooth stack, that is carved in stone.

It's not really the bug that is a concern, it's the software testing coming across as weak. Autoranging is not rocket science. You flowchart and test at the boundary conditions such as a range change, and a range change gone wrong. And this issue is only a problem for people working in the 600 ohm world it seems.

A repair depot in North America would not burden customers with the crazy shipping costs/customs paper work to Taiwan. It seems to be that ____ exclusivity agreement blocking that. I think they're playing in dangerous waters, with no formal sales/distribution a recall would be a disaster.

I haven't ever been bitten by Fluke multimeter bugs, and see they dealt with them as a warranty repair.

As much as I agree with you, Brymen does not sell in the USA so any meters imported (not unter greenlee or EEVBlog brands) are grey market and therefore it's not Brymens responsibility to cover service or shipping to get them fixed. That's a risk we pay for purchasing grey market goods.

[EEVblog]

And what should that response be?
Fluke for example very quietly fixed it in production units about 12 months later and made no mention of it at all to new or existing customers. A customer would have no idea what version they were buying.

A good first step is to take interest in the issue to understand it's impact to the customer and the root cause. After that the response should be commensurate with the impact. Ex. If it's a safety issue then there should be a recall. If it's an edge case that has a material affect on a small number of users it should be handled as warranty on a case by case basis. If it's an oddity quirk that does not have a material effect on end users fix in new.

Fair enough.
And where would you rank an autoranging hunting issue? For me it's the latter.

[EEVblog]

If Brymen is scared then let authorized dealers or a repair depot do it. Oh wait there's none in North America.

And who's going to pay for all the shipping and time?

Fluke had top quality firmware, it was tested thoroughly, there is no comparison in that regard.

https://www.eevblog.com/forum/testgear/fluke-87v-87-5-87-v-gsm-interference-fluke-says-firmware-flash-fixes-it/

I used to have a 70 series Fluke at work that had an autoranging problem when meanruing mains transformer primaries, it just kept autoranging, displaying nothing.

Is your position that the firmware is carved in stone, users have to suck it up if there are (non-safety related) bugs, you are stuck with the version you bought?

That's how almost every multimeter on the market is, very few have firmware update ability, and if they do like in this case, or the case of Fluke's for example, it's a factory/dealer thing only. Been like this since time immortal.
In an ideal world users would be able to update their own firmware, but I understand why many manufacturers are against this.
As a dealer, I would be out of business if everyone decided to return their meter for a firmware update and I had to pay the cost of shipping both ways. Same goes for the manufacturer.
Best to deal with it on a case by case basis.
If you are unhappy with your meter or it's firmware then I'm happy for you to return it and I'll refund your money.

[tautech]

Best to deal with it on a case by case basis.
If you are unhappy with your meter or it's firmware then I'm happy for you to return it and I'll refund your money.

Would this also apply for the last 2 replies here ?
https://www.eevblog.com/forum/testgear/brymen-bm-235-startup-issues/

[NoMoreMagicSmoke]

And what should that response be?
Fluke for example very quietly fixed it in production units about 12 months later and made no mention of it at all to new or existing customers. A customer would have no idea what version they were buying.

A good first step is to take interest in the issue to understand it's impact to the customer and the root cause. After that the response should be commensurate with the impact. Ex. If it's a safety issue then there should be a recall. If it's an edge case that has a material affect on a small number of users it should be handled as warranty on a case by case basis. If it's an oddity quirk that does not have a material effect on end users fix in new.

Fair enough.
And where would you rank an autoranging hunting issue? For me it's the latter.

In my day job I have to make these type of calls when they come up, so my take on this is definitely more tempered than others that are commenting here. Without full information I am also hesitant to make a definitive statement either. Without knowing the root cause of the issue I can not say for sure which category this issue would fall into. My gut is telling me this is likely a low impact issue, but again it could just be one symptom of a bigger problem somewhere that might place this into a higher impact category.

If it is simply an auto ranging quirk that only affects resistance measurement in one magic count value then, yes, I would put this in the latter category as well.

That being said, as a consumer that owns a BM789 of course I want my meter to be the mythical and unobtainable perfect and without flaw!   

[joeqsmith]

Best to deal with it on a case by case basis.
If you are unhappy with your meter or it's firmware then I'm happy for you to return it and I'll refund your money.

Would this also apply for the last 2 replies here ?
https://www.eevblog.com/forum/testgear/brymen-bm-235-startup-issues/

Wait.... I have a 121GW that is also having a few problems!!   

[EEVblog]

Best to deal with it on a case by case basis.
If you are unhappy with your meter or it's firmware then I'm happy for you to return it and I'll refund your money.

Would this also apply for the last 2 replies here ?
https://www.eevblog.com/forum/testgear/brymen-bm-235-startup-issues/

Any faults would be covered under warranty.

[NoMoreMagicSmoke]

It's not really the bug that is a concern, it's the software testing coming across as weak. Autoranging is not rocket science. You flowchart and test at the boundary conditions such as a range change, and a range change gone wrong. And this issue is only a problem for people working in the 600 ohm world it seems.

I think you are underestimating the complexity of that simple statement. I tested my meter, and I found the following:
- My meter ranges up when the 66000 count threshold is reached
- My meter ranged down when the 61000 count threshold is reached
- My meter shows 6xxxx when the resistor is applied above the 63100 count reading
- My meter shows 06xxx when a resistor is applied below the 63100 count reading

None of these values correlate with the other users "magic values" that cause this issue which range from 65000-67000 counts. Now which "boundary condition" should they be testing to?

I expect that this bug is probably related to some perfect combination of hardware tolerance and firmware logic. It is highly probable that they did test the expected boundaries but the hardware in the test meters did not hit the magic values to cause this issue. Additionally, it is possible that they did test what they expected to be the boundaries, but the boundary might be slightly different than expected due to some tolerance related variance.

Yes, a company should test the boundaries. The problem is it is very likely that the test engineers performed extensive testing, but they missed that one magic permutation that caused some unfortunate bug to exist.

There is no complex product in existence that is without bugs. The important part is does the device have a bug that significantly affects it fit for use and end user satisfaction. Equally important (as I have said before) is how the company responds and what they do when a bug is discovered.

[floobydust]

You decide to autorange up or down, on Ohms function. This is the boundary to test, in both directions that I'm talking about.
It involves changing the current-source (likely a resistor switched) and possibly the PGA and to a different (range) calibration factor, all at the same time (I don't have the Brymen chip datasheets to peruse but this is typical).
SURPRISE! This leaves you needing to range change in the opposite direction - which leads you to changing range again etc. and you see it's oscillating back and forth. As I've mentioned if you measure Ohms test current (using another DMM) while playing around with the potentiometer you should be able to see the changing/hunting. It could show up in other places at those boundaries as well.
No programmer expects to get stuck in a loop lol, that's why they have piles of hysteresis i.e. 66,000 vs 61,000 counts you mention.
Firmware algorithms around this can simply count the number of consecutive range changes done to see the hunting, the code itself can see what users are seeing and make an adjustment. It might be related to that range's calibration factor- unique to each multimeter. It can be a time delay needed to be added for settling time. Many hardware issues can cause it- but the point is it's too bad the bug is not easily fixed and slipped through testing.

[joeqsmith]

There is no complex product in existence that is without bugs. The important part is does the device have a bug that significantly affects it fit for use and end user satisfaction. Equally important (as I have said before) is how the company responds and what they do when a bug is discovered.

I have heard back from Brymen and they explained in detail the problem as well as their corrective action.  Indeed, some meters will not exhibit the inability to converge. 

[NoMoreMagicSmoke]

There is no complex product in existence that is without bugs. The important part is does the device have a bug that significantly affects it fit for use and end user satisfaction. Equally important (as I have said before) is how the company responds and what they do when a bug is discovered.

I have heard back from Brymen and they explained in detail the problem as well as their corrective action.  Indeed, some meters will not exhibit the inability to converge.

Good to hear. Are you able to share their response?

[joeqsmith]

Are you able to share their response?

This will require a change to the firmware.  In the future I hope to do a short clip showing the 789 before and after this update.

After switching from 600Ω Range to 6kΩ Range, firmware used the first hi-speed AD conversion data to deduct "600Ω Range Offset" to check if further Range-Switching was in need. In case "600Ω Range" and "6kΩ Range" have significant offset difference, the unit may not be able to converge the measurements at around 640Ω ~ 660Ω region in auto-ranging operation mode. Thus not every unit will have this bug. Only the unit with extreme offset difference case has this bug.

[NoMoreMagicSmoke]

Are you able to share their response?

This will require a change to the firmware.  In the future I hope to do a short clip showing the 789 before and after this update.

After switching from 600Ω Range to 6kΩ Range, firmware used the first hi-speed AD conversion data to deduct "600Ω Range Offset" to check if further Range-Switching was in need. In case "600Ω Range" and "6kΩ Range" have significant offset difference, the unit may not be able to converge the measurements at around 640Ω ~ 660Ω region in auto-ranging operation mode. Thus not every unit will have this bug. Only the unit with extreme offset difference case has this bug.

That makes sense. It also likely means that the root cause is an unexpected condition as a result of a manufacturing tolerance stackup.

I am really suspicious if it isn't a direct result of:

FYI

78608 version is only to enlarge allowance range for OHM hardware offset to reduce production failure rate. It does not affect meter performance.

Which would mean that the statement of "does not affect meter performance" isn't exactly true.   

The following is pure speculation, but I can certainly see how this can happen. R&D designed the meter and developed their tests around the design. The engineers may have even developed the original specifications to ensure that this scenario could not happen. After they start production the are seeing a higher than expected reject rate, and the process engineers determine that they can increase the tolerance to resolve the issue. The tests may have passed due to the test developers designing the bounds conditions around the original design, which might now no longer be the correct bounds conditions.

Regardless of how they got here. Based on Brymen's response to your previous inquiries, I would not be surprised if Brymen is currently in the process of developing new production change procedures to be absolutely certain this can't happen again.

[NoMoreMagicSmoke]

Are you able to share their response?

This will require a change to the firmware.  In the future I hope to do a short clip showing the 789 before and after this update.

After switching from 600Ω Range to 6kΩ Range, firmware used the first hi-speed AD conversion data to deduct "600Ω Range Offset" to check if further Range-Switching was in need. In case "600Ω Range" and "6kΩ Range" have significant offset difference, the unit may not be able to converge the measurements at around 640Ω ~ 660Ω region in auto-ranging operation mode. Thus not every unit will have this bug. Only the unit with extreme offset difference case has this bug.

Since I have a meter that does not exhibit this behavior, are you willing to compare your meter with the issue to mine?

- My meter ranges up when the 66000 count threshold is reached
- My meter ranged down when the 61000 count threshold is reached
- My meter shows 6xxxx when the resistor is applied above the 63100 count reading
- My meter shows 06xxx when a resistor is applied below the 63100 count reading

I am curious how close your transition threshold is to the offending reading.

[joeqsmith]

At 649.53 ohms it is stable.  Add one more ohm and it will hunt until 0.6759 where it is once again stable.   I'm sure it would change depending on the direction and range.   

[NoMoreMagicSmoke]

At 649.53 ohms it is stable.  Add one more ohm and it will hunt until 0.6759 where it is once again stable.   I'm sure it would change depending on the direction and range.   

Interesting. Does it only oscillate when going up? Or does it happen when going down as well?

[joeqsmith]

I think it cleaned switched (no oscillation) going back down and the threshold was a different value all together. 

[AVGresponding]

One wonders if the same mod as applied to the 869S for the same problem (on a higher range, granted) might work to prevent this?

[EEVblog]

For those interested in the exact details here:

uses hi-speed AD conversion data to do Auto-ranging mechanism. After switching from 600Ω Range to 6kΩ Range, firmware used the first hi-speed AD conversion data to deduct "600Ω Range Offset" to check if further Range-Switching was in need. In case "600Ω Range" and "6kΩ Range" have significant offset difference, the unit may not be able to converge the measurements at around 640Ω ~ 660Ω region in auto-ranging operation mode. Thus not every unit will have this bug. Only the unit with extreme offset difference case has this bug.

Our engineer has modified the firmware to fix this bug. After switching from 600Ω Range to 6kΩ Range, the update firmware version uses the first hi-speed AD conversion data to deduct "6kΩ Range Offset" instead to check if further Range-Switching is in need.

[NoMoreMagicSmoke]

Our engineer has modified the firmware to fix this bug. After switching from 600Ω Range to 6kΩ Range, the update firmware version uses the first hi-speed AD conversion data to deduct "6kΩ Range Offset" instead to check if further Range-Switching is in need.

[/quote]

Well, that blowa.a hole in my theory. That sounds like a straight up copy paste typo...

[pazuwu]

Did Brymen say in which firmware version was this corrected?

[J-R]

Couldn't reproduce the issue on my 78905 at any of the range transitions.

~655.xx does show as 0.622xx for a full second before settling on 0.655xx.  Also noticed the same behavior if you switch to mV and back at that approximate value.

[joeqsmith]

For those interested in the exact details here:

uses hi-speed AD conversion data to do Auto-ranging mechanism. After switching from 600Ω Range to 6kΩ Range, firmware used the first hi-speed AD conversion data to deduct "600Ω Range Offset" to check if further Range-Switching was in need. In case "600Ω Range" and "6kΩ Range" have significant offset difference, the unit may not be able to converge the measurements at around 640Ω ~ 660Ω region in auto-ranging operation mode. Thus not every unit will have this bug. Only the unit with extreme offset difference case has this bug.

Our engineer has modified the firmware to fix this bug. After switching from 600Ω Range to 6kΩ Range, the update firmware version uses the first hi-speed AD conversion data to deduct "6kΩ Range Offset" instead to check if further Range-Switching is in need.

You've been reading my emails again...

[joeqsmith]

Demo of firmware version 78911 which addresses the auto ranging problem mentioned earlier. 
 

[EEVblog]

Demo of firmware version 78911 which addresses the auto ranging problem mentioned earlier. 

Did they give you the chip programmer?

[AVGresponding]

Demo of firmware version 78911 which addresses the auto ranging problem mentioned earlier. 

Did they give you the chip programmer?

Surely they will have just sent an updated chip or meter?

[EEVblog]

Did they give you the chip programmer?

Surely they will have just sent an updated chip or meter?

He said it's the same meter, so either a new chip or he's got a programmer now.

[Chalcogenide]

I was looking at the Youtube video's comments and in a reply OP states "I changed the IC on the one I demonstrated." so it's fair to say that Brymen did not provide him a programmer but just a pre-programmed IC to replace in his meter.

[floobydust]

I thought Joe was blowing up the DMM IC U1, not the MCU U16?
Doesn't matter really, the firmware is cast in stone unless you want to ship it across the ocean. Brymen could have trusted agents in North America do F/W upgrades, repairs... or maybe not due to their obligations as an OEM which prevents them from selling here.
Could look at the Andes programmer but you'd still need the binary.

[c0d3z3r0]

@joeqsmith @EEVblog is 78911 the most recent firmware?

I got a new BM789 from @Welectron with FW 78908  They have no newer fw in stock and can't update the firmware. So, I'm sitting here with a shiny new DMM with buggy firmware....
They said they'll contact Brymen and I might send it to them (Brymen) for update. Not really what I want to do with a new device, tbh.
I hope they'll find a customer-friendly solution.

EDIT: Just to clarify, I don't want to blame Welectron for that Problem, I'm sure they'll give their best to solve this :-)

[NoMoreMagicSmoke]

Do you actually have any issues with your meter? I have an identical meter to yours, and mine does not demonstrate the resistance glitch.

[EEVblog]

@joeqsmith @EEVblog is 78911 the most recent firmware?

I got a new BM789 from @Welectron with FW 78908  They have no newer fw in stock and can't update the firmware. So, I'm sitting here with a shiny new DMM with buggy firmware....
They said they'll contact Brymen and *I* might send it to them (Brymen) for update. Not really what I want to do with a new device, tbh.
I hope they'll find a customer-friendly solution.

It honestly isn't a big deal. It's almost certain that you'll never encounter the bug, and if you do then it's obvious and you can just switch to manual range.
This is how every Brymen meter and most other brands and model have worked since time immemorial.

[EEVblog]

Do you actually have any issues with your meter? I have an identical meter to yours, and mine does not demonstrate the resistance glitch.

Yes, only some meters exhibit the problem. Brymens response as to why this is the case has been posted.

[c0d3z3r0]

Well, my BM789 simply shows 0 Ohms then. I wouldn't call that *obvious*. Also, when pressing "RANGE" it sometimes simply turns off completely...

[NoMoreMagicSmoke]

Well, my BM789 simply shows 0 Ohms then. I wouldn't call that *obvious*. Also, when pressing "RANGE" it sometimes simply turns off completely...

That's something completely different than the bug that was recently fixed. Does it ever change from 0 ohms?

[Monkeh]

Well, my BM789 simply shows 0 Ohms then. I wouldn't call that *obvious*. Also, when pressing "RANGE" it sometimes simply turns off completely...

Err, that is not the described fault, and shutting off when changing range is not normal either.

[c0d3z3r0]

Well, my BM789 simply shows 0 Ohms then. I wouldn't call that *obvious*. Also, when pressing "RANGE" it sometimes simply turns off completely...

That's something completely different than the bug that was recently fixed. Does it ever change from 0 ohms?

I don't think so. For me this happens around 660 Ohms. Yes, it changes, when either changing the resistor value or when range switching works - each 3-4th doesn't...

[c0d3z3r0]

Well, my BM789 simply shows 0 Ohms then. I wouldn't call that *obvious*. Also, when pressing "RANGE" it sometimes simply turns off completely...

Err, that is not the described fault, and shutting off when changing range is not normal either.

A different symptom, but probably the same bug

[NoMoreMagicSmoke]

Well, my BM789 simply shows 0 Ohms then. I wouldn't call that *obvious*. Also, when pressing "RANGE" it sometimes simply turns off completely...

That's something completely different than the bug that was recently fixed. Does it ever change from 0 ohms?

I don't think so. For me this happens around 660 Ohms. Yes, it changes, when either changing the resistor value or when range switching works - each 3-4th doesn't...

This is very interesting. Any chance of posting a video?

[joeqsmith]

I thought Joe was blowing up the DMM IC U1, not the MCU U16?
Doesn't matter really, the firmware is cast in stone unless you want to ship it across the ocean. Brymen could have trusted agents in North America do F/W upgrades, repairs... or maybe not due to their obligations as an OEM which prevents them from selling here.
Could look at the Andes programmer but you'd still need the binary.

As for blowing up ICs with both the Brymen BM786 & BM789, I suspect that thin Canada air causes hallucinations.   I pushed the prototype BM786 to failure (12kV peak) in the following video and damaged Q4.  The set was replaced and I continued my testing of the meter.  Because the two meters share the same front end, I only ran the BM789 on the 6kV generator.   Personally I consider meters that have protection circuits that limit the amount of damage to allow repairs a big plus.   Consider that the $600+ Keysight meter I just looked was damaged beyond repair at 5kV.     

Damaging the Brymen BM786 prototype at 12kV and then repairing it by replacing two transistors:


Damaging the Keysight U1282A at 5kV and taking out several IC, some custom. 

[joeqsmith]

Well, my BM789 simply shows 0 Ohms then. I wouldn't call that *obvious*. Also, when pressing "RANGE" it sometimes simply turns off completely...

Err, that is not the described fault, and shutting off when changing range is not normal either.

A different symptom, but probably the same bug

Agree with Smokey the Bear, make a short video showing the behavior.   If you have found a new problem and I am able to replicate it, there is a good chance we could work with Brymen to address it.   

@joeqsmith @EEVblog is 78911 the most recent firmware?

As of three weeks ago when I posted that last video it was.  I'm not a distributor and am in no way affiliated with Brymen.  The only way for you to know for sure would be for you to ask them.   

[c0d3z3r0]

Well, my BM789 simply shows 0 Ohms then. I wouldn't call that *obvious*. Also, when pressing "RANGE" it sometimes simply turns off completely...

Err, that is not the described fault, and shutting off when changing range is not normal either.

A different symptom, but probably the same bug

Agree with Smokey the Bear, make a short video showing the behavior.   If you have found a new problem and I am able to replicate it, there is a good chance we could work with Brymen to address it.   

Sure, I made a video now. There are two things you can see 1) weird jumping of the measure value after a second or so and 2) now random resetting of the DMM when switching through ranges, instead of turning off o.O
https://www.veed.io/view/261c54f7-b4a1-4e5e-89cc-e05c4cb08251

Fun fact... I am unable to reproduce the 0 Ohms readings anymore. I should have filmed that earlier . However, the resetting behaviour got worse as you can see. This *only* happens around 66x Ohms (666...  ).
1 happens regardless of the resistor value, but seems unusual to me and I never saw such thing with other DMMs, what do you think? Here's another video of that:
https://www.veed.io/view/e9a4ccda-ac7c-4aba-a357-9880bcae5435

@joeqsmith @EEVblog is 78911 the most recent firmware?

As of three weeks ago when I posted that last video it was.  I'm not a distributor and am in no way affiliated with Brymen.  The only way for you to know for sure would be for you to ask them.

Thank you. I already sent them a message. I guess that will take some days.

[floobydust]

Pressing the Manual range button might just be aggravating a bad connection inside. See if it's the rotary switch, try applying some pressure, a gentle squeeze to the switch instead.

[floobydust]

I thought Joe was blowing up the DMM IC U1, not the MCU U16?
Doesn't matter really, the firmware is cast in stone unless you want to ship it across the ocean. Brymen could have trusted agents in North America do F/W upgrades, repairs... or maybe not due to their obligations as an OEM which prevents them from selling here.
Could look at the Andes programmer but you'd still need the binary.

As for blowing up ICs with both the Brymen BM786 & BM789, I suspect that thin Canada air causes hallucinations.   I pushed the prototype BM786 to failure (12kV peak) in the following video and damaged Q4.  The set was replaced and I continued my testing of the meter.  Because the two meters share the same front end, I only ran the BM789 on the 6kV generator.   Personally I consider meters that have protection circuits that limit the amount of damage to allow repairs a big plus.   Consider that the $600+ Keysight meter I just looked was damaged beyond repair at 5kV. [...]

In your Brymen BM786 DMM Part 3 video you mentioned getting a new MCU and putting that in, I assumed it got damaged at 12kV.

Did they give you the chip programmer?

Surely they will have just sent an updated chip or meter?

He said it's the same meter, so either a new chip or he's got a programmer now.

He appears to be replacing the MCU, but the air is thick 

[joeqsmith]

In your Brymen BM786 DMM Part 3 video you mentioned getting a new MCU and putting that in, I assumed it got damaged at 12kV.

Maybe just watch the first minute of Part 3 where I explain the reason for the change.   

[joeqsmith]

Sure, I made a video now. There are two things you can see 1) weird jumping of the measure value after a second or so and 2) now random resetting of the DMM when switching through ranges, instead of turning off o.O
https://www.veed.io/view/261c54f7-b4a1-4e5e-89cc-e05c4cb08251

Fun fact... I am unable to reproduce the 0 Ohms readings anymore. I should have filmed that earlier . However, the resetting behaviour got worse as you can see. This *only* happens around 66x Ohms (666...  ).
1 happens regardless of the resistor value, but seems unusual to me and I never saw such thing with other DMMs, what do you think? Here's another video of that:

I only saw the one value in the video.  Odd it wouldn't reset like this with other values as it seems like a mechanical problem.    Sorry but I wasn't able to replicate it with the meters I have.   

[Monkeh]

Well, my BM789 simply shows 0 Ohms then. I wouldn't call that *obvious*. Also, when pressing "RANGE" it sometimes simply turns off completely...

Err, that is not the described fault, and shutting off when changing range is not normal either.

A different symptom, but probably the same bug

Agree with Smokey the Bear, make a short video showing the behavior.   If you have found a new problem and I am able to replicate it, there is a good chance we could work with Brymen to address it.   

Sure, I made a video now. There are two things you can see 1) weird jumping of the measure value after a second or so and 2) now random resetting of the DMM when switching through ranges, instead of turning off o.O
https://www.veed.io/view/261c54f7-b4a1-4e5e-89cc-e05c4cb08251

With that behaviour I would simply go to Welectron and have it replaced.

[NoMoreMagicSmoke]

Well, my BM789 simply shows 0 Ohms then. I wouldn't call that *obvious*. Also, when pressing "RANGE" it sometimes simply turns off completely...

Err, that is not the described fault, and shutting off when changing range is not normal either.

A different symptom, but probably the same bug

Agree with Smokey the Bear, make a short video showing the behavior.   If you have found a new problem and I am able to replicate it, there is a good chance we could work with Brymen to address it.   

Sure, I made a video now. There are two things you can see 1) weird jumping of the measure value after a second or so and 2) now random resetting of the DMM when switching through ranges, instead of turning off o.O
https://www.veed.io/view/261c54f7-b4a1-4e5e-89cc-e05c4cb08251

Fun fact... I am unable to reproduce the 0 Ohms readings anymore. I should have filmed that earlier . However, the resetting behaviour got worse as you can see. This *only* happens around 66x Ohms (666...  ).
1 happens regardless of the resistor value, but seems unusual to me and I never saw such thing with other DMMs, what do you think? Here's another video of that:
https://www.veed.io/view/e9a4ccda-ac7c-4aba-a357-9880bcae5435

@joeqsmith @EEVblog is 78911 the most recent firmware?

As of three weeks ago when I posted that last video it was.  I'm not a distributor and am in no way affiliated with Brymen.  The only way for you to know for sure would be for you to ask them.

Thank you. I already sent them a message. I guess that will take some days.

I agree with he others that said get a replacement. Looks like you light have gotten a dud

[c0d3z3r0]

Greetings to all. I have a question about measuring resistance with this multimeter. At first it shows a slightly lower value, after a second the final one. Is it so for everyone? Firmware 78908.In reviews, he showed the value immediately.

Couldn't reproduce the issue on my 78905 at any of the range transitions.

~655.xx does show as 0.622xx for a full second before settling on 0.655xx.  Also noticed the same behavior if you switch to mV and back at that approximate value.

So, I am no the only one...

Pressing the Manual range button might just be aggravating a bad connection inside. See if it's the rotary switch, try applying some pressure, a gentle squeeze to the switch instead.

As I said, this *only* happens around 660 Ohms.

I only saw the one value in the video.  Odd it wouldn't reset like this with other values as it seems like a mechanical problem.    Sorry but I wasn't able to replicate it with the meters I have.

Check the second video. There's two values, as described by J-R.

How can a mechanical problem be tied to a specific value? ;-)

Anyway, I alread had contacted Welectron. Let's see what they come up with next week.

Just to clarify, I don't want to blame Welectron for that Problem, I'm sure they'll give their best to solve this :-)

[joeqsmith]

I only saw the one value in the video.  Odd it wouldn't reset like this with other values as it seems like a mechanical problem.    Sorry but I wasn't able to replicate it with the meters I have.

Check the second video. There's two values, as described by J-R.

How can a mechanical problem be tied to a specific value? ;-)

Both videos show 664 ohms.  There was nothing shown that suggests the meter would behave differently with another value.   If it is tied to a specific value as you suggest, I would suggested leaving the leads open or shorted to show that. 

So yes, it appears like a mechanical problem to me. 

[c0d3z3r0]

I only saw the one value in the video.  Odd it wouldn't reset like this with other values as it seems like a mechanical problem.    Sorry but I wasn't able to replicate it with the meters I have.

Check the second video. There's two values, as described by J-R.

How can a mechanical problem be tied to a specific value? ;-)

Both videos show 664 ohms.  There was nothing shown that suggests the meter would behave differently with another value.   If it is tied to a specific value as you suggest, I would suggested leaving the leads open or shorted to show that. 

So yes, it appears like a mechanical problem to me.

The second video *clearly* shows 0.6401, then 0.6639 - exactly as both J-R and Antrus already described.

Ahh, I see, you don't believe anything you haven't seen on a video. Got it.....
Then watch this - no mechanical problem! https://www.veed.io/view/1b8810ab-a6ca-44b4-90ac-27d78e7bf110

(Now you tell me, that you don't believe it's one and the same DMM? *scnr*)

[joeqsmith]

Notice you removed the leads, adding one more variable to the mix.  Also the technique changed along with where you were pressing.   True, I don't know what firmware was installed on this meter.  Was it indeed different?   

[floobydust]

If it's not a mechanical problem, it might be the old F/W has a problem with the act of selecting manual range (mode) when the auto-ranging it hasn't yet settled or it converged at the seam, and then it appears to reboot?

[joeqsmith]

If that's true, he could have tried a 100 and 1k or so to move it away from that switch point but still give roughly the same settling time.     

[c0d3z3r0]

If that's true, he could have tried a 100 and 1k or so to move it away from that switch point but still give roughly the same settling time.   

I already said that this doesn't happen for other values. I don't have to prove everything with a video, though.

[c0d3z3r0]

@joeqsmith Regarding that second issue (jumping value), your BM789 with 78908 shows the same behaviour. Just look at your own video
https://youtu.be/siPjGNL6U0E?t=92
The dmm first shows 0.6358k, then after a second switches to 0.6998k.
This is gone with 78911 later in that video.

[armandine2]

I don't have to prove everything with a video, though.

The video is a representation of what has happened, presumably - as you yourself have just demonstrated with your analysis of Joe's video.

or am I missing something

[joeqsmith]

Sorry,  I don't see the problem. 

***
Are you expect meters will always settle on one read?   

[AlexTee]

Just noticed, Welectron is now offering paid firmware update service for BM78x series: https://www.welectron.com/Brymen-BM780-Firmware-Update_1

Has anybody tried this out so far? Specifically, interested in whether the DMM persists its factory calibration after their update procedure?

[EEVblog]

Has anybody tried this out so far? Specifically, interested in whether the DMM persists its factory calibration after their update procedure?

Yes it keeps the factory calibration.
They would be using the exact same tool and process that I have.

[giosif]

Hi,

Just got my BM789 with a plan to make it my go-to DMM, but I've noticed a few "oddities" (in no particular order):
1. When in Diode mode, sometimes, if I short the probes, I get a short beep, followed by a pause of 1-2 seconds, and only then I get the continuous beep and flashing backlight.
   The other times, the meter starts beeping continuously and flashing the backlight right away.
   In fact, you can kind of see this behaviour in my video below, when playing with the rotary switch.
   Not a big problem, but can still a bit annoying from the perspective of inconsistent behaviour, when you are troubleshooting something.

2. Low battery indicator - as per the video, the threshold for this is different for the different functions.
   For example, I did some measurements and, when in ACV measurement mode, the threshold for displaying low battery is approx. 3.76V.
   At the same time, the threshold for displaying low battery when in DCV mode is 3.58V.
   Is this intended, I wonder (i.e. Brymen feel confident the meter will display accurate DCV readings even with lower battery levels?)?

3. Rotary selection switch - in my brief time spent with the meter (got it only a few days back), it already happened that I wanted to switch to Diode mode and, without connecting the probes to anything, the meter starting beeping as when measuring a short.
   When I looked a bit more carefully, I noticed the rotary switch had ended up between functions (namely, between Capacitor/Diode and Temp).
   Once I set the switch to the correct position, everything went back to normal.
   In the video, I tried to replicate this issue but I had to explicitly try to get it into an intermediate position and try a few times in order to succeed.
   Subsequent attempts seemed to be easier to replicate it, though.
   In any case, given I encountered this under normal use once already, and this in less than a week since I got it, I am concerned it will become a recurring problem.
   Has anyone else encountered this and, if so, how often does it happen to you?

In short, I want to love this meter, but the above, and especially point 3 above, rather puts me off.
Am I being too picky here?

Thanks!

Link to the video - https://youtu.be/kEJgoO0yBYo

[giosif]

[...]
2. Low battery indicator - as per the video, the threshold for this is different for the different functions.
   For example, I did some measurements and, when in ACV measurement mode, the threshold for displaying low battery is approx. 3.76V.
   At the same time, the threshold for displaying low battery when in DCV mode is 3.58V.
   Is this intended, I wonder (i.e. Brymen feel confident the meter will display accurate DCV readings even with lower battery levels?)?
[...]

Adding to the above, just noticed this mentioned in the manual for the BM789:
"Low Battery: Below approx. 3.7V"
So, that means there is an issue with the meter not displaying the low battery indicator in DCV (Ohms and others as well) until battery level reaches about 3.58V.
Maybe this was addressed in fw version 78911?
Anyone willing to check?

[J-R]

I have a 78905.

1) I can reproduce the diode issue if I turn the DMM on, go straight to that mode and short the leads right away.

2) Low battery behavior is a complex issue IMHO, especially when you consider the discharge curves of various chemistries and how they act under load.  Then throw the backlight into the mix and it's a bit of a minefield for detection.  But on the flip side, in normal use the backlight will probably cause the low battery indicator to come on first, so that may be the most consistent indicator.

3) Yes, I have had the issue with the Brymen selector switch ending up between positions.  Some years ago my BM235 did that very badly when I turned it off and somehow ended up between Off and AutoV.  I put it away for a while and some weeks later the plastic had malformed and the switch literally flopped in the breeze.  I reversed the process over another few weeks and that fixed it.

Since then, I have been taking apart my Brymen DMMs and adding a small amount of lubricant to specific areas of the selector switch mechanism. I also add a tiny amount of precision lube to the contacts on the PCB.  Between the two, the selector feels a lot nicer to me, but maybe that is personal preference.  Due to the short warranty and overseas shipping costs, I consider all my Brymen DMMs to be disposable.


If you just purchased your BM789, it might already have firmware 11.  What version is yours?  (Turn on while holding ▲.)

[sonpul]

1. A 0.5 second interruption occurs in BeepLit™ Diode Alert operation. I work with the device daily and intensively. If you had not paid attention to this moment, I would not have noticed it.

2. I use 3xAAA Panasonic Eneloop Pro batteries, BM789 lasts a very long time with them. The battery icon is present when measuring V~ and mV dBm. The measurements are accurate, but I'm just heeding this nutritional warning. Maybe in these truerms modes, the nominal voltage is desirable for the converter.

3. I can create a beep by switching between diode continuity and temperature. But in thousands of measurements, I have not set the switch in this position.

BM789 is so good and convenient that you stop paying attention to these "shortcomings"

[mwb1100]

(I don't have a BM789 but a BM786)

2. Low battery indicator - as per the video, the threshold for this is different for the different functions.
   For example, I did some measurements and, when in ACV measurement mode, the threshold for displaying low battery is approx. 3.76V.
   At the same time, the threshold for displaying low battery when in DCV mode is 3.58V.
   Is this intended, I wonder (i.e. Brymen feel confident the meter will display accurate DCV readings even with lower battery levels?)?

It would not occur to me that a low battery indicator showing in some modes but not others when the battery is at the edge of a low voltage condition would be considered a problem unless readings were wrong.

3. Rotary selection switch - in my brief time spent with the meter (got it only a few days back), it already happened that I wanted to switch to Diode mode and, without connecting the probes to anything, the meter starting beeping as when measuring a short.
   When I looked a bit more carefully, I noticed the rotary switch had ended up between functions (namely, between Capacitor/Diode and Temp).

I can't recall ever landing between settings.  But if you believe this might happen again more than once in a a blue moon (or once in a lifetime), then you might want to consider an exchange or different meter.  I'd guess that landing between positions accidentally was a fluke though.  But there has been selector oddness discussed before (things like pressing on the selector would cause the meter to misbehave, and if I remember right, Dave did a video diagnosing a BM786 problem that turned out to be a defect on the the selector's PCB contact area).  I believe those problems were one-off defects rather than problems with the BM78x design.

[Neutrion]

1.
FW version 08 here. The beeplit has this short pre-beep also but not with a one second brake, but rather maybe a tenth of a second. And yes sometimes it doesn't prebeep, but I never noticed this when working with it. You want to keep the probes connected to have a good contact and to read a stable value so I think this is not an issue.
2. See explanation from J-R! Some mode draws more power, so the battery goes suddenly to a lower voltage. Happens only when the battery is getting close to the treshold level. The exact value depends on the battery type, and temperature.

3. The Brymens switch is rather too stiff to get it accidentaly betwen position. Now I tried it, it is indeed possible, but you really have to try it hard.

I would suggest to Brymen to actually make the switch be much lighter, it would feel better when not working in heavy gloves.

[2N3055]

3) Yes, I have had the issue with the Brymen selector switch ending up between positions.  Some years ago my BM235 did that very badly when I turned it off and somehow ended up between Off and AutoV.  I put it away for a while and some weeks later the plastic had malformed and the switch literally flopped in the breeze.  I reversed the process over another few weeks and that fixed it.

Since then, I have been taking apart my Brymen DMMs and adding a small amount of lubricant to specific areas of the selector switch mechanism. I also add a tiny amount of precision lube to the contacts on the PCB.  Between the two, the selector feels a lot nicer to me, but maybe that is personal preference.  Due to the short warranty and overseas shipping costs, I consider all my Brymen DMMs to be disposable.

I have BM525S and BM869S and you simply cannot put them between the positions, it snaps very positively into predefined positions...
I think your opinion is very judgmental, based on sample of one. I'm not saying it did not happen to you, but one is not a sample to make conclusions on. I have no experience with other models, though.

Also, shipping and such is a USA problem. In EU they are readily available and affordable.
Here Fluke is more expensive than in USA.

Price difference between new F87 and BM869S (that is feature vise closer to F287 minus the graph mode) is such that you can buy 2-3 BM869S here... That takes care of potential warranty issues. And in last few years I had BMs, they are going strong...
I consider them very good quality and excellent alternative to super expensive Fluke so far.

[J-R]

I have about 10 Brymen DMMs and about 10 Fluke DMMs.  I have taken both types apart to tinker with.  The Fluke selector switches feel superior to me, partially because they are larger and easier to grab, but also they generally just nicely and gently fall into position.  Personally, the 121GW selector switch is my favorite.

It's true that there is not really a systemic "problem" with the Brymen selector switch, but I'm not going to lie and not post my experience with the BM235!

[giosif]

Thank you all for your input!
I do appreciate it.

Trying to answer some of the points raised:
1. My FW version is 10 (also shown in the video I included with my original post).
2. With regards to the occasional delay in Diode mode, again, more of a slight nuisance rather than a significant issue.
3. About battery level indicator, I should have mentioned that my voltage level measurements were done with my bench power supply powering the meter.
   So, the different voltage levels I talk about are not due to the different meter functions drawing more energy than others.
   And my main "issue" with it is more related to the reliability of the readings for those measurements which do not display the low battery indicator (while others do).
   Said differently, if I turn the meter straight to DCV measurements and I do not see a low battery indicator showing, then I want to *know* I can trust the DCV readings the meter is giving (i.e. the readings could not have been impacted by a low battery level).
   Otherwise, this can be a safety hazard, in my opinion, unless Brymen comes out states they do this intentionally (i.e. they are confident the DCV readings are reliable even below 3.7V and down to 3.58V or so).
4. With respect to the rotary switch landing between functions, maybe I've been spoiled with the likes of Gossen and Fluke, where that just doesn't happen. Not in my experience, anyway.
   I've had similar incidents with some Agilent/Keysight DMMs but, even with those, not within the first few days of using them.
   In any case, I'll try to exercise the switch for a while as with normal use (i.e. without explicitly trying to land between functions) and see if I encounter the problem again.
   Maybe it was just bad timing with the first occurrence.

[mwb1100]

  Said differently, if I turn the meter straight to DCV measurements and I do not see a low battery indicator showing, then I want to *know* I can trust the DCV readings the meter is giving (i.e. the readings could not have been impacted by a low battery level).
   Otherwise, this can be a safety hazard, in my opinion, unless Brymen comes out states they do this intentionally (i.e. they are confident the DCV readings are reliable even below 3.7V and down to 3.58V or so).

I don't see a statement about this specific detail in the manual; in fact, I don't see any statement about low battery behavior other than the one you already pointed out: "Low Battery: Below approx. 3.7V".  So you will need to ask Brymen for such a statement if you haven't already.

Alternatively, you could experiment with reading known values at low battery levels to see if readings go bad before the meter dies (my hope would be that readings remain correct even when the low battery symbol is displayed until the meter actually powers off due to low battery voltage).

[giosif]

I don't see a statement about this specific detail in the manual; in fact, I don't see any statement about low battery behavior other than the one you already pointed out: "Low Battery: Below approx. 3.7V".  So you will need to ask Brymen for such a statement if you haven't already.

I've asked Brymen and the short version of their response is this behaviour is as intended.
I suggested they document this in the manual, so people know it's not a defect.

[Antrus]

Hi everyone. My bm789 stopped measuring temperature. Can anyone post the contents of 24cs04 on the board with the buttons? The calibration is stored there.

[EEVblog]

2. Low battery indicator - as per the video, the threshold for this is different for the different functions.
   For example, I did some measurements and, when in ACV measurement mode, the threshold for displaying low battery is approx. 3.76V.
   At the same time, the threshold for displaying low battery when in DCV mode is 3.58V.
   Is this intended, I wonder (i.e. Brymen feel confident the meter will display accurate DCV readings even with lower battery levels?)?

Very likely intentional, as the True RMS converter might require extra voltage headroom.
You should treat all multimeters the same when it comes ot the low baterry indictor. i.e if the low battery light is on then take any measurements with a grain of salt.

[mwb1100]

There are several in depth reviews of Brymen meters (among many other brands) on https://lygte-info.dk/info/DMMReviews.html

The reviews include measurements of when the battery low indicator came on and at what point readings became unstable.  The list of Brymen meters reviewed doesn't include the BM78x series, but for the five reviewed two (BM27s and BM869s) remained stable until shutdown or an error message is given.  The others (BM235, BM525s and BM829s) gave what seems to be quite a bit of buffer between the voltage where the battery low indicator comes on and the reading become unreliable.

As I said there is no data for the BM78x series in the reviews, but in general it looks like Brymen takes care to show the battery low indicator well before the meter starts giving questionable readings.


BM235: Meter works down to 1.7V where meter turns off, battery symbol show at 2.5V
       Reading is stable until about 1.8V where it increase significantly (5.000V -> 5.206V)
       
BM27s: Meter works down to 2.3V where meter turns off, battery symbol show at 2.5V.
       Reading is stable until meter turns off.
       
BM525s: Meter works down to 4.0V where it says "InErr", battery symbol show at 6.8V
        The meter reading is stable down to 5.7V on battery.

BM829s: Meter works down to 4.0V where it says "InEr", battery symbol show at 6.7V.
        The meter reading is stable down to 5.6V, then it will show too high value (It was 36% to high at 4V)

BM869s: Meter works down to 5.6V where it says "InErr", battery symbol show at 6.3V.
        The meter reading is stable until it reports "InErr".

[ru_tash]

what is the voltage reference chip inside BM789?

[NoMoreMagicSmoke]

what is the voltage reference chip inside BM789?

The chipset has the same package and pinout as a Hycon technologies HY3131. If that is indeed the chipset used it contains an internal voltage reference.

[ru_tash]

what is the voltage reference chip inside BM789?

The chipset has the same package and pinout as a Hycon technologies HY3131. If that is indeed the chipset used it contains an internal voltage reference.

thank you for reply, if HY3131 then chip's internal reference has 70ppm/С drift.
BM869s has stand alone reference REF43G with 25ppm/С